We tend to struggle a lot with keeping up our membership and attendance so I end up playing whatever spec is necessary, often playing all three specs in an ICC lockout. A few months ago after spending the first eleven bosses as Holy, one of our tanks disconnected and didn't come back. I had to run grab my Protection gear from the bank. I got back and we started the encounter. About two seconds before The Lich King engaged, I realized I was still specced Holy. Needless to say, that attempt did not last too long.
Failing to notice accidentally hitting your 'reply' bind while DPSing and freaking out over vent, claiming your keyboard is broken and clicking abilities for the last 20% of a fight.
Only to later have your GM start to laugh and link the following in raid chat: 22222qqqqeee33223224ffffff222211122233333444433333
I can't even count the number of times I've gone into Rotface in 10 man without putting RF back up after tanking Fester...