I got the following error the other day (it has been shortened...):
"the web part you attempted to add no longer exists in the closed web parts gallery."
This was a pretty disturbing error...there was no real apparent cause for this problem...nothing in the logs. After messing about with it for a while I figured out that it was a dll version in the GAC that was causing the problem. On my dev box I had an updated version but the version in our prod envs is different.
It would be nice if the server was a little more helpful with its error messages....HTH if you come across it
