The biggest problem with undo is that sometimes allowing the user to recover from a particular action is also undesirable behavior. If I accidentally typed my u

The biggest problem with undo is that sometimes allowing the user to recover from a particular action is also undesirable behavior. If I accidentally typed my username and password into a file, I want it to go away, right now. There's a somewhat worrying trend with applications such as Facebook and Google Apps where software is more concerned with making users believe it's doing what they need it to do, and less concerned with doing what users need. I'm not saying this necessarily crosses the line - if I have the space on my server I'm storing lots of incremental backups. But there's always a trade-off. And some people might rather be assured that when they hit delete that it goes away forever than that they never lose anything they meant to keep.