Does archiving already archived messages means deleting them? I am using Mail.app using an imap-account (not Gmail).
Just as it should, the app created an 'Archive' folder where it put all the messages in on which I used the archiving function.
I was browsing this Archive folder when, out of habit, I used the archiving function again on one of the conversations. This caused the conversation to disappear and I cannot find it anywhere: Not in any folder, the trash, anywhere on the imap server or the Mail.app or its cache.

How can this be intended behavior? Is this a known bug (I could not find any mention of this online). Where are my messages?

Update

I contacted Apple support. This turns out to be a known bug. Look out.
I just tried this in a non-Gmail account and the message has vanished! It is in none of my folders, including the trash. You can use Time Machine on Mail to get your messages back. Also, I suggest reporting this bug to Apple http://apple.com/feedback/mail.html

(In Gmail accounts, the message goes away for a few minutes until the next time Mail synchronizes with the server.)