I have a slightly-larger-than-generated Savanna village with a LOT of villagers in it, and they all act normally.  Except when trading with Cartographers, after a few simple exchanges, the entire server will crash.  This could be after a few panes of glass, or reems of paper, but i can't tell if it is related to hitting a new level or not.  I tried to do a single trade at a time to see when the the server freezes up, and it seems like it happens after 4 to 6 exchanges.

So this CAN stop leveling from happening, but I've also seen it where the server went down (which would normally back rev my progress a bit), and then when i restarted, the first thing i saw were the leveling sprites being emitted from him.

As far as i can tell, only the Cartographer does this, and it's only happened to me in this densely populated village.