Many years ago, I took Tae Kwon Do, and I was starting to get bored from the school *. So one day during the exercise portion of the class, instead of doing jumping jacks, I did a "jack". I didn't do the leg part at all, and needless to say, the Sah Bum Nim wasn't at all pleased :P I slacked off a lot during the ass end of my Tae Kwon Do experiences (if you're out there reading this, realizing who I am, I'm sorry, sah bum nim!).
* (IMO, the head instructor was very talented and respectable, but his dojang was getting a bit too commercial for my tastes; nothing wrong with that if you're into that sort of thing, it's just my personal preference)