We've seen this issue across a couple of projects now, and it seems to affects most versions of Sitecore 6.
If you notice that random users will sometimes be set as the name in the "Updated by" field when they haven't edited the item, then you've stumbled across the same bug in Sitecore we did.
This is triggered by using the “My Items” or “Unlock All” functionality in Sitecore. When this unlocks items, it does it in a Sitecore Job as it can be a long running action.
"Sitecore jobs are executed in different threads without the context user. So this job is executed under the anonymous user or user from the thread (the thread can have another user)."
There is a fix, unfortunately you'll need Sitecore support to build you a dll for your specific Sitecore version - so give them a shout :-)