The co-worker in question is a librarian. The time of the event is three minutes after the closing of the library. The librarian is part time, and of the chair-filling variety- so not one I particularly respect, but neither of the sort to have done noticeably irritating things over the years.
Until the event in question at which point he went into the record books!
Most librarians stay to clear the library after closing, but not all of them, and not this sort of librarian. I was scheduled to stay to empty and secure the library myself, and we had just done it. The doors were all locked, the bathrooms checked, and the upstairs "all clear" had been given. At this moment the librarian in question decided to leave the library through the front doors.
The staff do not, as a rule, leave through the front. We have a back door. But whatever his reason he needed to leave through the front. Fine. Perhaps he was walking or heading south. The doors are crash door locked, that is they are locked from the outside, but if you are inside they will always unlock with a push to let you out.
He pushed the door to exit.
As he did so two groups, totaling six people, were approaching the closed library. Instead of this librarian squeezing through the door and making sure it closed behind him, or instead of telling any of the people approaching "I'm sorry, we're closed." this librarian calmly threw open the doors and walked obliviously away, leaving the door easily accessible for everyone arriving. Six people poured into the library.
I don't really think I need to say anything more.