Playing online creates its own breed of mistakes. sometimes players are 'multi-tasking' and concentration lapses causes mishaps. Mis-clicking is, I believe, the most common one. Here's one from the highest level.
I was watching part of one of the weekly Cayne matches on BBO the other Sunday - a real treat. The matchup I kibitzed was Helgemo-Helsness vs Fantoni-Nunes. One one deal, Geir had this hand:
Partner opened 1S and Geir responded 2C (red/white) when Fantoni overcalled 2D. Tor jumped to 4D (splinter) and Geir trotted out Blackwood. Fantoni wasn't done and came back in with a re-raise to 6D! Tor passed, showing odd number of key-cards. What would you bid?
Double seems kind of obvious, knowing that slam can't make when partner only has 1 ace. Helgemo passed! Which was a better decision than doubling as it turned out that 6D was cold!!
Who would have thought that 6S was a winning sacrifice?
Pass looked really weird, even if you sometimes get the feeling that Geir really can see through the backs of the cards. Checking the evidence (emailing Tor), it turned out that Geir indeed had mis-clicked, intending to double.
You got to pick your occasions for those.