Unintentionally using Push to Talk while sneezing or coughing. Apparently, everybody needs to hear it. XD
On my monkcher, my Cocoon is hotkeyed to 5 and my Abundant Step is on 6. So I'm standing in front of Lolth taking damage and thinking "I'll heal myself to save the healers some SP". One wrong button later, I get a facefull of Lolth's bellybutton, and then featherfall to my death. Twice. And I still refuse to switch those hotkeys around!