Really, really, really unlikely to happen, but anyway:

Attach a file to a post -> get distracted for 3 hours - in the meantime the forum admin deletes the "orphaned attachment" -> try to submit the post ->

General Error
You are not allowed to access this file.

Could that be caught another way, maybe using a nicer error message?

((changed the 3 hours to 1 minute for testing, but that shouldn't matter))

Probably "Will not fix", but maybe someone searches for the error message in bug tracker sometime (if there's a better search function ...) and/or you forgot about that one (very unlikely, too )