>>33318981I've played through the whole game and never had a problem mega evolving. Was the rom played pre-patched or not? Mine was.
Anyway, other glitches include contests not working at all, and participating in the Frontier and Battle tents will wipe your tms/hms pocket and multiply everything in your items pocket to maximum quantity as well as fill in the remaining empty space with the last time you had. There is another frontier glitch but it's not game breaking.
Otherwise I've mostly ran into graphic glitches in battles that went away when I accessed pokemon summaries. It's pretty safe.