Never assume that the user will never encounter your crazy error condition. "I'll put this in a message box for me to see it" is what you might think. "They'll never see this in the field" is a tempting thought. But then your users might end up seeing something like this:

I can see a line number from a C++ file and some variables being printed to the screen, but what am I supposed to do with that? Is this error bad enough that I should reboot? Should I tell the vendor? Ok, now which application actually popped this error? Was it a browser? Outlook? or maybe Vista itself? I really have no clue. And if my mom ever got this error, she'd be even more lost than me.

So even though you're sure that the error condition in your if-else block will never happen and there's no way the user will ever see it, just remember this blog post.