Philosophy
'No Scotsman would do that' — until one does, and suddenly he's not a real Scotsman
Philosopher Antony Flew named this fallacy after an imagined exchange: someone claims no Scotsman would commit a crime, is shown a Scotsman who did, and simply redefines "true" Scotsman to exclude him. It's a way of protecting a generalisation from any counterexample by quietly moving the goalposts every time one shows up.