I'm a "handsy" speaker; that is, I gesture a lot. It's me, and it's natural, and I don't have a problem with it, but where this gets me into trouble is when I'm wearing a wireless lapel mic.
I forget it's there and find myself smacking my chest, rustling papers right next to it as I'm getting ready to give handouts, and any other variety of microphone faux pas.
Luckily, I've never kept mine turned on in the restroom, as two colleagues of mine have, by mistake.
Here's the latest one that someone else did (for a change). Standing at a lectern with a fixed mic, the person introduced the next speaker, then proceeded to clap right into the microphone inches away from her hands. Eardrum-shattering, but she didn't hear it at all.
It's funny that, when you're using a microphone to magnify your voice, you still have to project or you might not be heard, but when you just brush it or rustle a piece of paper against it, everyone can hear the noise!
Just like most things about public speaking, using a microphone requires a level of awareness beyond just what you're saying, but of everything your body is doing, so pay attention!
What other microphone mistakes have you made or seen?
For more on proper microphone use, see the following:
Don't be macho, use a mic
I can't hear you
How to use a microphone