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 PX41 (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.