Fart in an elevator

There are some things that are hard to hide. An earlier thread talked about the elephant in the room, and another would be a fart in an elevator. Any others come to mind?

Lady MacBeth's damned spot.

I don't know about a fart in an elevator - the perfume/aftershave some people wear and their personal hygiene frequently lead to mistakes being made - but 'blood on the moon' is often the phrase used to identify the unmistakeably obvious.