I don't know how many times I've been typing something when a dialog pops up mid-word and then it disappears as quickly as it appeared because my brain doesn't work fast enough to tell my fingers to stop typing the instant an unexpected dialog box appears. This is generally A Bad Thing since whatever popped the dialog thought it was important enough to disrupt whatever I was doing in the first place. This just happened with my firewall. I was in the middle of typing an IM and apparently some app wanted to use the internet. Which app was that, you ask? I have no idea because the dialog contains a default button.
I consider this a bug and I'm going to file a bug report when I'm done typing this. Please, friends, if you must pop up a dialog that steals focus, don't give it a default button. I and many others thank you.
Read: Focus-stealing dialogs should not have default buttons