Minions: Panic in the Mailroom (2013)

Two Minions (Ken and Mike) are working in a package room to send parcels through different parts of the lab, while Ken is busy playing a handheld video game. When a package containing expired PX-41 (the serum that El Macho used to transform the Minions and himself in Despicable Me 2) gets jammed in the pneumatic delivery system, it transforms Mike into an Evil Minion- but he keeps shifting back and forth, much to Ken’s amusement.