It takes weeks for the liquid nitrogen in a dewar to boil off actually. And they're stored upside-down so the head defrosts last in that scenario.

Read up on it and this or basically this has already happened a bunch of times.

Six months ago this would have been sarcasm but now, no. This is an extremely good point given what happened. I am still baffled how the janitor did that, could do that, knew to do that, and was even in a position to unplug.