That happens to me everytime I try to move an NPC that is "in transit", as in they are currently moving. Well, at least it did with yesterday's snapshot, but I submitted a bug report a few days ago. Were you moving people around?
One (known) cause of this 'crash' is hack-moving an NPC within the same chunk.
This is caused by a bug in the drag'n'drop code, and as such not an issue if you don't cheat. (I will take a shot at fixing it tonight, though)
Anyway, the reason that that assert is there is because we suspect things might occasionally go wrong there, and apparently they did in this case.