No software is ever more important than the user. It should not ever, for any reason, steal focus from the user. It's especially stupid and counter-productive to steal focus while the user is typing to put up a dialog box with buttons that can be activated by keys the user is pressing. Windows is especially bad about this. I've seen many flashes of dialog boxes, with no idea what they said because I happened to hit Enter or the Space Bar or whatever when the dialog box flashed up.

Apple Mail is occasionally misbehaved in this way. Sometimes it has trouble writing a message to the "drafts" folder on my IMAP server (*shakes fist at dovecot*), so it helpfully interrupts my typing (sometimes once every minute or so) to tell me this. You know what? I don't care! Not right now, anyway. I was typing a message! Please be quiet and let me do the thing I'm trying to do. Let me know about the error in some more unobtrusive way and I'll deal with it later.