Cancelling a mass page move corrupts database

One of our new recruits accidentally moved our user guides home page and all child pages to another space. Mid way through the move process, he realised his mistake and clicked cancel - the effect was disastrous.

Some of the pages were moved to the target space, but the source space (our user guides) was left completely empty - or so it seems. When we tried moving the page tree back to the user guides space, it says that some pages already exist (yet we can't see them) and asks to rename them. We tried renaming but it just keeps coming up with the "some pages already exist" error and so on.

In short, cancelling a mass page move leads to lots and lots of pain. If it's cancelled then it should stop in a stable state where we can move pages back to the source space.