Has been happening on every Ubuntu machine I've used to date, which is a total of 4. Started in 12.10 as far as I know, but it might have happened with 12.04, 12.10, 13.04, 13.10 and now 14.04. It doesn't seem to matter what I'm doing, but what always seems to trigger it is opening a web browser or some other application first from the sidebar. Firefox was was the most recent trigger. Instead of opening my browser, which it acts like it's going to do... the session locks, goes to the login screen, and won't unlock with the correct password. By 'won't unlock' I mean it unlocks then immediately locks again without first restoring unity, it does not produce 'incorrect password' I suspect this is a virus or password snooping software because of the fact it won't unlock with correct password information and I think if this IS a security issue, it should be fixed asap considering it's widespread throughout multiple versions. It's probably not a virus, but it is certainly suspicious behaviour to see your pc do that... wouldn't you think?