Here's a good example of a real world usability problem. See that sign on the door? It says "please use this door". Seems straightforward right?
But think about it - have you ever seen that before? No, you always see "please use OTHER door". As a result, every time I go to open that door I use the wrong one.
What does this tell us? 1. People don't read your signs. 2. Life is easier if things fall in line with your pre-existing expectations. 3. Quiznos, realize that 99% of the time people are reading other doors, and none of their signs say that. So I don't care if you like your way better, even if it was logically better in some way, it still fails.
Extra credit: next time you're anywhere that some kind of transaction takes place (like a cash register) look at how many paper signs are plastered all over the place. Each one is a sign that some expectation is being broken. And none of them get read.