Last week I was teaching and we were taking a about dimension of data and I made the "that means your data is demented" joke. The problem was I had said nearly the same thing the day before in a different (but related) discussion. Most folks groaned a bit but one guy was as quick as a whip and said:
"You have to use select distinct joke from jokes, you told that one yesterday."
We all roared laughing and class was on hold for several minutes. I still chuckle about it now.
Good one mate!