so I am not going to do it, I do not own any t8 lamps, nor do I want to. but lets just say I put 4 foot t8 lamps, i believe they are 32 watt? in a fixture with a magnetic ballast that is supposed to run 40 watt t12 lamps. would they have the same effect as the 34 watt energy saver t12 lamps, would they be over driven, or would they just not work?

T8s are kind of a retrofit for T12s, so if you install a 36w T8 in a 40W magnetic gear there won't be any problem, same for 18W T8 in a 20W gear. I think that if you stick a 32W T8 in 40W magnetic gear the tube life will be slightly shortened.

It depends on the type of F40 magnetic ballast.
(this is for 32w T8's .. and rapid start, though preheat should be similar):
If its a LPF ballast, the lamp and ballast will be underdriven. Neither will be harmed. You'll just have a dimmer light that will also be unlikely to even start in cold weather.

If its a HPF ballast, the lamp and ballast will be overdriven. Really won't hurt the lamp much, but might cause the ballast to overheat & thermal-protection shutdown. It'll also likely have a slight 60hz flicker..

For 36w T8's, they'll work fine for 1-lamp preheat(that's what they're designed to run on)
But won't do well on 2-lam preheat
And most rapid start magnetic simply won't light them at all, but if you coulg get it to go I would assume it would be:
LPF ballast, lamp and ballast underdriven (not as much as a 32w T8) So no harm done.
HPF ballast, might actually run close to normal.
((I should expirament with it sometime .LOL. )

i've repeatedly encountered exactly this setup at my work, as many of my coworkers are of the "it's the same length and it lights up, what's the difference?" mentality. if it's a newer magnetic ballast, with F34T12 bulbs printed on it as compatible, you should see completely normal operation without any issues. there is a particular fixture i can think of which i had thought contained a newer electronic ballast, but upon pulling the cover off i found such a magnetic ballast. i was impressed with how quickly the bulbs start, and how normally they run, on that ballast... originally i was going to go fetch some F34T12s out of the light bulb room, but the t8s seem so happy that i just left them in peace.

now, on older ballasts, before the introduction of F34T12 bulbs, you may see slightly more mixed results. some of them don't mind, and some are even less happy with an F32T8 than they are with an F34T12. i think power factor/ballast factor has a lot to do with it, as mentioned in xmaslightguy's response... really, experimenting is the best way to find out. just don't use any bulbs or ballasts you're extremely concerned about damaging, that's always a good rule of thumb when playing around!

 In most cases 32 watt lamps have higher arc voltsge and lower running current 270 mA .  Ive ran them on preheat ballast
It will correct it self run them around 330-350 mA.  Meaning the 32 watt lamp is over driven and most cases the ballast doing less work cause 32 watt higher arc voltage lamp. making them run cooler on preheat.  On rapid start it can be very hard to start as it being around 200 to 220 OCV while lamp arc voltage is 101-105’for the T12 40 watts.  And the T8 32 watts is 127 volt at 270 mA