At home... bit too much alcohol, playing with a power supply and forgetting to check the 115/230v selector. It makes a nice bang and smells a bit when it dies.
At work... deleted wrong directories from *nix boxes on numerous occaisions, and once dropped a load of tables from the
wrong Oracle instance (not my fault... both instances used the same username and password, and I just kinda forgot to check my ORA_HOME and ORA_SID variables
)