I've got one. There's a bit of a story to this one. I used to have an almost OCD-esque habit of listing all the different types of enemies that appeared in each level or area of my favorite computer games. One day I was reading off one of these lists aloud (the game was called Bugdom 2), and discovered the following tongue twister:
Fish, Flies, Fleas, Frogs
It might not be a particularly difficult tongue twister on its own, but try saying it with perfect pronunciation five times in a row quickly. Neither my brothers nor I have been able to do it. :blink: