The phenomenon of playing a game on Steam
and either getting killed or missing vital information due to Steam blocking out the bottom-right corner of the game to tell you that a friend has logged into a different game. Named for mailman Cliff Clavin from the TV show "Cheers," who often spouted useless, wrong, and/or unwanted trivia to the annoyance of the people around him.
"FUCK, I'm dead! I would've SEEN that landmine if I weren't being Steam-Clavined just then!"