a real howler
1. A mistake, error, or blunder that causes great laughter at the expense of the person who commits it, i.e., one that results in "howling" laughter. Do you remember when John actually slipped on a banana peel? That was a real howler.
2. An extremely funny joke, anecdote, or remark that results in "howling" laughter. Boy, Jerry sure knows some real howlers. I thought my sides were going to split I was laughing so hard.