[DiapDealer]

You could be right. It does't really affect me at all (mostly because I almost never have url frags in my ebook links).

I just worry about new users making a dog's lunch of things without realizing it. I know we can't save everyone from themselves, but I can't really see any downside to giving them a heads-up that they're about to do something risky, either.

Yes the problematic link is technically broken already, but it will still get a reader to the right file even if the target id doesn't exist.

I think the way the software currently works is fine, too. I just don't think a "here's what can go wrong IF..." warning could hurt either. It's a pretty drastic procedure to be kicked off with no confirmation.