Predicting user behavior is a fruitless time-consuming task. I get the gmail example... but the quitting example? How are you supposed to know that this time

Predicting user behavior is a fruitless time-consuming task. I get the gmail example... but the quitting example? How are you supposed to know that this time they really meant to quit and didn't just slip and click the wrong button? This is why I hate GUIs. I'm too clumsy with a mouse to trust it. How many times have I opened the wrong application from an applications menu and had to wait for it to startup only to shut it down right away? How many times did I close the application I was using by accident and have to open it again? I get a lower error rate on a keyboard using a tiled window manager. So... should I code a new feature, fix a broken one, or spend time trying to predict what mistake (or intentional action) you are going to make next? Priorities...