I can't explain why you saw that error message the first time, but I'm guessing every other time it was because the account already existed. There are, to my knowledge, only a few possible causes for that error, and most of them involve apocalyptic scenarios where the site's database shouldn't work at all, so that satisfactorily keeps the risks down.

When you open a thread on the forums, you don't get sent to the last page, but to the "end" page, which contains the last 20 posts. This was Torco's idea, and now that the interface reflects it properly, generally a good thing. This was done to minimize the chance that the last post on a page would be missed by readers when the page number flipped over.