Reversal of expectations

 Some jokes work through reversal of expectations. 

I've been counting calories, in and out, for a week, trying to be more mindful of what I'm eating and how much I'm exercising.   

So far I've gained two pounds.  [I'm not sure it's funny, but it has the structure of that kind of joke at least.] 

Seinfeld says: Pop tarts can't go stale because they were never fresh in the first place.  

***

The famous joke about the restaurant.  Two women are complaining: "The food there is inedible, it tastes like garbage; it makes me sick, etc... " and the other one adds, ",,, and the portions are small!"

A famous musician says: If I don't practice for a week, I know it.  If I don't practice for a month, my audience knows it. If I don't practice for six months, the critics know it.  [Usually found in a less funny version, with the critics second and the audience last.]. 

A pianist friend of my father's had a whole list of things to say to a colleague after a not-so-great performance, like "I've never heard the piece played that way before!" "That was a performance I won't forget any time soon!""I have no words."  I cannot remember what the quips were now, but they all could be said for either a great or godawful concert.  

Lester Young had a joke: "What Lester plays, Stan Getz."  Puns are usually bad, but this one is a zinger.