Well I was waiting for this to happen and it finally did.  I work at the library and we have tons of MH can lights, some are open (70w) others are closed (100w). Well one recently went out above one of the staircases, and to me the lampo looked funny, a bit longer than normal.  It was replaced today and I got ahold of the old lamp, only to find out that it was an unprotected lamp that had been put in an open fixture. The lamp is pretty black, so I'm glad it didn't explode or anything.  The guy that replaces them now is very good about making sure to get the protected lamps, so I know he wouldn't have put it in there.

For that fixture to be open rated, it's supposed to have a special socket that can only accept protected MH lamps with the thin neck. That's the code imposed by the National Electrical Code. Another loophole I know of are the CMH track lights, they have standard sockets which makes it very easy to install enclosed-rated lamps. All CMH PARs are open fixture rated, but E-17 lamps can still screw in. I've seen this at the local supermarket and it bothers me a lot!

Thats what I thought as well.  The bulb was noticably lower in the fixture compared to the others, I'm wondering if the socket has been tampered with?