Like you see the sign over the soup tureen at the buffet place, and the sign says "New England Clam Chowder". You lift the lid, and inside you find something that looks like chili. New England clam chowder is white because it is milk-based. Chili is brown because it is beef based. They don't look anything like each other. It is just that someone put out the new soup and didn't bother to change the sign, and there isn't a manager walking around to notice that sort of thing.
ETA additional questionage:
What are some examples of incorrect signage that annoy you?
What are your non-signage pet peeves?