Undoing typing on iPhone
This bugged me for a while until I decided to google it. Every now and again, I would get a popup on my iPhone 4 saying "undo typing" or "nothing to undo," and I couldn't figure out what it was.
Apparently, when you shake your iPhone, it acts like an undo button on your PC. It brings up that menu, and you can click "undo typing" to get rid of the last thing you typed. I find it handy if I want to erase a text and start again.
(For os v3.0 or higher)