Every few days I accidentally click Report Spam when I really meant to click Archive. Now, to Google's credit, they do offer a simple, obvious undo path for these accidental clicks. But I can't help wondering why it is, exactly, that these two buttons with such radically different functionality just have to be right next to each other.
I think it's worth mentioning that the keyboard shortcuts for "archive" and "Report spam" are extremely different from each other. In fact, the destructive ones (report spam, delete) require me to be holding shift down at the time to use them. So that's nice. (insert US-centric keyboard shortcut rant here, of course.)
Also, as far as I've noticed, just about every operation in gmail is immediately undoable with the exception of 'send', and there's an addon buried in the settings page that lets you make send undoable as well.