Speaking of Undo, one of google's common features across several of it's products is the use of undo instead of confirmation, I absolutely LOVE this. If I archive an email in gmail (app or browser) it doesnt't ask me if I am sure I want to do that, but it does give me the option to immediately undo that action. This little feature is so wonderful when you think about it. It allows you to complete an intended action in one step, but abort an unintended action in two steps. That is great functional design.