Mentioning that I work in the oilfield about 30 seconds before the girl starts cutting my hair. Nothing like getting a haircut while listening to a tirade about the price of gas. I walked out of there bewildered by my momentary stupidity and amazed that both ears were in the same place they were 15 minutes before.

My second favorite: Sitting in a Business Class seat in BKK, getting ready for a nice long multiple-leg journey home to IAH after a month away from family. Here is the conversation with the guy next to me:
Him: So what brought you to Bangkok.
Me: I work in the oilfield, how about you?
Him: Oh, I'm a policeman. I just save people for a living...

Cue iPod...

From then on, the reply is just "I'm an engineer". Always works as a conversation ender anyways.

Lawyers, quit feeling sorry for yourself. At least you aren't Petroleum Engineers!