I actually intended this to be part of MBS-253, but it doesn't seem to have been implemented as I expected.

If you enter a barcode that's not valid, you're prevented from going any further in the release editor. That's a bad idea because covers do have mistakes and most people don't have barcode scanners to check the real barcode.
We do want to ask the user to confirm that yes, they really want to add an invalid barcode (like we do when editing release events on the mason server), but there should still be a way to enter them (like the mason server's checkbox).

Here's an example where the barcode on the cover is wrong: http://musicbrainz.org/release/d084e488-6418-40d8-9c97-81cbf7e10fa4.html The first digit (not the check digit!) is missing and I would have had no idea what the correct barcode was if I hadn't been able to scan the barcode myself.

I can't find an example right now, but I would imagine that the release editor would also prevent you from editing a release which already has an invalid barcode, even if you're not touching the barcode.

Tickets complaining about the mason release editor doing the same thing:
http://bugs.musicbrainz.org/ticket/3437
http://bugs.musicbrainz.org/ticket/2982
http://bugs.musicbrainz.org/ticket/5789