Type a website address incorrectly while at work so that “Sexy Personals” pops up on your screen in full view of your supervisor.
Say to someone dressed as a clown for Halloween, “Oh, your shoes are too big..” implying they go well with the costume. Unless you know that yes, in fact they ARE too big.
Forget to shut the baby gate on your incredibly tall staircase so that your little niece goes tumbling half way down before you get to her. [FYI – if you happen to do this, don’t panic. In my experience the baby is always fine.]