Given my OS version this may not be a common issue, but running 1.7.9 for the first time I found my entire inventory emptied completely. Could be a big annoyance if you had any valuable stuff...

Bug title updated. Please reopen.

The issue is NOT that 14w11b->1.7.9 loses your stuff; that's known and documented. The issue is that you get an automatic 'upgrade' from 14w11b->1.7.9 WITHOUT BEING ASKED, and if you don't spot it, you'll get an unanticipated data loss. This could affect anyone whose launcher profile is set to 'Use latest version', if they don't realise what the consequences of the changeover will be BEFORE they start playing - so it's very easy to get caught out.