For some reason I woke up this morning (okay, afternoon) with this old sketch from The State running through my head. It's really a great piece and the perfect example of how to pull off a one note joke. There's really nothing wrong with a one note sketch if it's A) funny and B) done right. The death trap that most one note joke sketches fall into is that they go on way too long, lumbering about with no discernible structure or exit plan. It doesn't matter how funny a joke is, people tune out if it doesn't seem like it's going anywhere.
This State sketch skirts all of those pitfalls; it's short, it's well structured with a clearly defined beginning middle and end, it plays its joke to the hilt, and then it gets out. Also, it repeatedly uses the word "poop" which is always hilarious.