Opening a parachute but to no effect.
Realising you did not pay attention to your instructions on your exam paper and forgot to answer BOTH questions.
Putting your hand in your back pocket to pay your tab at a bar to discover you never took your wallet.
Having your johnny burst during private time with the missus.
Looking at the black guy using the urinal next to you, who is incidentally twice your size, with you dressed for a party as a grand wizard of the K.K.K.