Same here with guys that used E27 24V lamps on 240V in a staircase

push button - BAFFFF on several floors at once

I have a 12V 50W lamp that is in a standard B22d base and which looks like a 230V lamp. If you put it in a regular socket you get a flashlamp that takes out the mains.

With regard to the long extension cord, that is why they are limited in length to 30m, as longer ones will have too high a voltage drop at rated load anyway. I did wire an outbuilding with 10mm split concentric cable where I used 30m, running at it's rated 40A supply breaker. Underground cabling in PVC conduit, even though the cable is rated for direct burial, but I am wary of the digging ability of gardeners, so the conduit is extra protection. Replaced the original cabling which was supplying the same load, but which was a 2.5mm SWA cable.

@SeanB:
Even that replacement is not clean, running cable rated for 40A and direct burial in a conduit, while still using 40A breaker mean you are asking for cable overheat and fire.
The conduit thermally insulate the cable, so it can not dissipate as much heat as it is designed for, so it should be derated by at least one step (so max 25A breaker in your case of "40A" cable).

The cable is rated for 40a when buried in conduit, and is more voltage drop limited than anything else.

Generally here you find a supply to a newer house is 10mm, with a 60A breaker, whilst for older houses you will find a 16mm cable with a 80A breaker. The street side invariably is protected with a 100A fuse, more to protect the supply cable than for load cable protection. Supply cabling will be 16mm bare copper or 25mm Aluminium ABC in most cases, here it is only in rural areas that you will find a pole pig and 11kV lines on the same pole. Overhead 11kV is generally on a separate set of poles, and in urban areas it is almost alway buried, with either dedicated substation buildings or a GRP minisub on a concrete pad. The minisubs are generally added as the load increased over the years, or were there for new developments that were higher density and thus would overload the old supply lines. The 11kV cabling is almost always run at a very low current to reduce drop, this gives a good reserve margin for load increases.

I sorta did one today. The 14 watt fluorescent desk lamp I won on eBay came today. The cord is rubber, hard and brittle and had crumbled where it goes into the base of the lamp. The seller rightfully was afraid to test it.  I was brave and made sure wires didn't touch and it works. I will be replacing that cord shortly.

It has an inside etch GE CW with something loose inside that scratched the phosphors up but it shows no wear.


http://www.ebay.com/itm/250937600896?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

found a good one:
in my school, on a sloped ceiling, 10' above the floor on the "low" side and over 20 on the "high" side, there are rows and rows of surface-mount troffers for 2 t12 lamps. $ foot long and like 8 inches wide. Awhile back they were changing bulbs and 2 fixtures FELL on the guy. Turns out the fixtures are mounted into dead air behind drywall (not to rafters) with those fasteners that expand when you put them in. Just drywall holding a 150lb row of 4 fixtures, and the heavy magnetic rapid start ballasts. Just heard this today.

On the minisubs, you often see broken cabinets where illegal connections have been made by izinyoka. Scary is that they have been making connections to the incoming supply, and running cable ( loosely speaking that is, it is anything that will in some way conduct electricity to some degree rather than what is generally regarded as electrical cable or cord) to supply power, often whilst working live and whilst drunk  to multiple locations. The flash marks are very visible on the GRP housings.

Here are some REALLY stupid (and dangerous) ones...
On a boat I am restoring (with other people involved, won't get into that here), there was AN ELECTRICAL FIRE!
The original wiring was lead braided outer cover, with cloth wires inside. (this was 32v DC) Parts of it had been redone in "romex"-looking stuff, that looked more modern. Other stuff was spliced in with lamp "zipcord".
I will post some pix.
The AC wiring was REALLY bad...
The 240v output out of the 15KW diesel generator went to a breaker panel that looked pretty modern... and so was all of the 240v stuff, right? But where it split off into 120v, that's where things got nasty:
The main disconnect was 60 amp vintage cartridge fuses, which went to screw-in fuses.
Some of the outlets were still 2 prong
Some of the wiring looked pretty modern, but some was VERY OLD! Some more of that lead-jacketed wiring I mentioned earlier
That fed 2 prong outlets, at least the newer stuff fed grounded outlets
VERY DANGEROUS!
The fire started thanks to the DC wiring (32 volts of DC is really nasty), and also damaged the 120VAC stuff.
GUESS what is currently taking its place (but hopefully not for long)
Wiring hooked up to the 15KW with NO BREAKERS OR FUSES! My Dad put that in just to have power, which is OK for a couple outlets I guess. They were fed by extension cord wire.
But guess what the new owner's crew added???
More outlets, some in boxes, some just sticking off the end of the BX cable that they're using. Ground wire not even hooked up to the outlets, and not even taped off or having a wirenut on the end.
The BX cable is single-stranded wire, which clearly violates the things I have been told, one being that single-strand wire is  VERY BAD, BAD, BAD, BAD, BAD, IDEA on boats. It will eventually crystalize and short out, likely causing a fire. Use multi-strand wire! DUH!
Three outlets (with boxes at least, but no faceplates) were on a piece of  scrap two by four, attached to an extension cord.
Several florescent lights (vapor-tight electronic F32T8 ones) were hooked up temporarily, with the BX cable, and wire-nutted into other stuff. No idea where they plugged in or where the spaghetti of loose wires got power from. Will get pix tomorrow if I can

That boat is long gone now...
Update: I have been doing stupid electrical experiements, including using a malfunctioning UPS to provide low voltage and trying a 12v bulb-didn't light at first, then a bright flash!
34v bulb didn't do anything
And others...

Wait.... The battery inside IS 12V why a bright flash happen ?

(note - the battery is on a FELV supply which mean that it can shock you when the UPS is plugged in, as it is 120/240V to earth)

Wait.... The battery inside IS 12V why a bright flash happen ?

The UPS is supposed to be loaded by the battery (to be charged), so the OCV is way higher than the 12V. The light bulb was of way less load than the UPS used for the battery charging, so the voltage went up, the lightbulb blew ouit. The charging control rely on the battery acting as a capacitor and measure the voltage when there is no current (e.g. during mains zero cross). If the voltage is low, it apply full charge current (without voltage control) till the next measurement time. And because the lightbulb voltage drop when there is no current, the charger see no voltage, so aapply full power for the next cycle.
This arrangement is there to assure the charging control respond exclusively to the chemical state of the battery, but not to any voltage drop on the inner resistances. And the measurement does not have to happen so frequently, as no reasonable charging current could change the chemical state (= the charge level and mode) in any significant manner within a few seconds, so good controllers check the voltage only once per 10 seconds and keep about a second delay between the current cut out and the voltage measurement point, so their response to anything else than the real chemical state is greatly suppressed. The 10 second is there to keep the average current not affected by the breaks necessary for the voltage measurements.
Simple chargers use the mains and SCR's properties to control that: When the battery voltage is below the threshold, the comparator turn ON the SCR. This mean current start to flow, the voltage may increase, but till the mains zero cross the SCR keep itself ON. So the next point the comparator is able to control the current is after that time, when there is again no current into the battery, so (except for some capacitance effects) the serial resistance is supressed from influencing the voltage measurement.

Okay, I've come across (and done) others since:
using wirenuts on multi-strand wire
175w mercury vapor area light in my backyard, fed by extension cord (not it's permanent home though and rarely on)
Using one F40T12 and one F32T8 lamp in F40T12 "shoplite" fixtures, with LPF ballasts. (because I didn't have enough T12s at the time;I've since fixed this with proper lamps except one fixture has one cool white and one warm white lamp. The cool white lamp is going bad/rectifying now so the whole thing strobes at 60/120HZ
On one fixture the T8/T12 combo thing worked, but one the one with the aforementioned bad lamp it flickered, but the cool white lamp was original. It looked odd with moxed diameters and one 3500K and one 4100K lamp.
Trying to ballast an EOL 22w circline with an incandescent lamp.  For the socket I used a table lamp and wrapped wires around the prongs on the plug.  I accidentally shorted it and ruined the lamp (but it was EOL and strobed on a preheat ballast anyway)
using strings of Christmas lights as extension cords
In a True Value hardware store (half of it lit by 8 foot (I think 75w slimline) fixtures there were many flickering/striating/rectifying (all at once, it looked like something else) lamps.  Sone had 60w "energy saver" lamps, some had full power 75w lamps, and I think a few had 95w lamps (all of various brands of course)  Most were flickering/buzzing to some degree. Loud enough to be heard clearly over the other music playing in the store. The ones over THE LIGHTING AND ELECTRICAL AISLE were buzzing/flickering (how ironc). Also, they has just completed remodeling.  In any case, though, I was happy to see they didn't get rid of the 8 footers (although the entry had F54T8HO lights with Altos in them).
Not dangerous, but unusual:
at Cape Decision Lighthouse at one point they had some radio equipment grounded by just running copper wire all over the rocks (and I think dead-ending in saltwater). Although whatever it protected is now long gone, if you look you will still find copper wire all over the place
This place also had some pretty amazing derelict wiring-VERY IMPRESSIVE! And lots of those old "SNAP!" light switches in use (possibly hooked up to knob and tube wiring?) Although some of it was new silent switches and Romex for sure.
I think that's all for now.

I have seen a lot of multiwire branch circuits (two hots, one neutral, one gnd) where both hots were on the same phase rather than on opposite phases and the neutral is burnt.

Some electricians dont take the neutral seriously at all anyway, even in circuits with just 1 hot....

Then you wonder why you get random voltages (not 120V but anything 0-240V, here not 240V but anything 0-415V) and stuff blows up

Some electricians dont take the neutral seriously at all anyway, even in circuits with just 1 hot....

Then you wonder why you get random voltages (not 120V but anything 0-240V, here not 240V but anything 0-415V) and stuff blows up

Multiwire branch circuits have their pros and cons
Pros: reduced voltage drop, fewer home runs from panel means fewer supplies and smaller raceways which means less money and time spent
Cons: neutral wire can be overloaded if both hots are on one phase rather than two,  if neutral termination becomes interrupted then both circuits could be putting out more than 240 volts (USA) and like you said ASH stuff blows up. 
My uncle is an electrician and I help him occasionally and I can't tell you how many violations we see.