You've all done it.
You did something horribly, horribly wrong and you hope no one saw it.
I'll start with mine.
Professor Putricide, 25m, third phase. A slime was still up and far away from the boss. When it died, I spammed my charge/intercept/intervene macro to get back to the boss. I miss-clicked the boss and clicked our main tank instead, intervened him, and got one-shot.
I was embarrassed beyond belief but it was hilarious in hindsight.
So, how about you?