When I give presentations, I often display the following slide:
This is what I call a joke grenade: you pull the pin, and it takes about five seconds before any laughter occurs. The delay occurs because the humor is subtle; it takes most folks a few seconds — and it takes some folks a great number of seconds — to realize that the third error is that the sign contains only two errors.
This joke derives its humor from self-reference. For a better example of self-reference, have a look at the Self-Referential Aptitude Test by Jim Propp. This is an exceptional puzzle and a lot of fun, but allow yourself between 20 minutes and 2 hours to complete it.
I completed the SRAT in less than 20 minutes.
Both of these statements are false.
For my money, Groucho Marx delivered the best self-referential joke of all time:
I would never join any club that would have me as a member.
Neither would I, Groucho. Neither would I.