I was wondering if my Banette use "trick" when is Mega evolved while holding banettite. His form will disappear instantly or "trick" will just fail?
Trick will just fail, because you cannot trick your mega stone, even if you are mega-evolved.Same for switcheroo and Knock Off.
Trick will fail.Mega stones can never be transferred by thief, covet,trick switcheroo,fling or bestow. Embargo and Magic Room also cannot prevent the use of Mega Stones to Mega Evolve Pokémon.
Source: Bulbasaur Wikipedia.