According to Bulbapedia the only differences in M/MissingNo's moveset is that MissingNo can use TM50 (Substitute) and M cannot, and MissingNo does not learn Pound. (
http://bulbapedia.bulbagarden.net/wiki/MissingNo)
The differences in movesets occur with the "non-standard" variety that are used to store image data for the two museum skeletons and the Pokemon Tower ghost. They inherit the TM/HM matrix of the Pokemon last accessed in the game's memory.
I also wouldn't necessarily call M a "gamebreaker", as far as I know everything stored on the cartridges from hex 00 up through the end of the trainers is relatively "safe". Everything after that, however, or forcing the game to read trainer data as a Pokemon, is where the game can crash and become corrupted (ZZAZZ glitch, "Super glitch" effect, etc). When encountering MissingNo/M the infamous "pause" before battle is the game accessing save data, but nothing is overwritten. Theoretically if you were to shut the game off while this happened it would ruin the game, but so would turning the game off while saving otherwise.
What you'd need to look out for are glitch moves and non-standard data being read as Pokemon. Neither M nor MissingNo learn any moves located outside of the game's standard library of attacks.
Nice touch with the mentioning of Bird type, though. I forgot to bring that up.
/
"M" (Missingno.) cool fact DYKG
