D3v has written up a very interesting guide for input leniency in Street Fighter 4. The special move shortcuts for the game have become pretty infamous with players, some who love them while others outright despise them.

To demystify what's going on behind the scenes, he examines which inputs it requires to make certain moves come out.

This is a very good read if you'd like to further understand why you'll sometimes get a Shoryuken when you didn't even come close to doing the standard motion.