As it turns out, a is only guaranteed to crash the game at:
However Level 13 is actually considered the Point Of No Return for game crashes. a has a chance to crash the game at every single level from then on. And it’s not just levels, at some point, nearly anything would be able to freeze the game and alter your save file! I highly don’t recommend using this Pokémon, even if it is a poliwhirl hybrid
Note:


I was using a kabutops form Missingno in this picture and a turned it into a Poliwhirl form Missingno.

And it wasn’t temporary.
Oh and also that pic has a different Missingno to the one I actually used against it. Nothing else became Poliwhirl. Just the 2 kabutops Missingnos I was using
Another Note: no matter what I said in the thread, both of the Poliwhirl hybrids are dangerous. It just didn’t say that in the bulbapedia article for the 2nd one