Continuity mistake: During the dance fight towards the end of the movie between Gru and Balthazar Brat the guitar/piano/laser thing swaps from a right-handed item to a left-handed one when Gru gets it.
Continuity mistake: When Bratt steals the diamond for the second time he breaks the window on the ceiling and glass starts to fall. Cut to Bratt and the falling glass disappears.
Continuity mistake: There had never been any signs of the ship being anywhere close to land, but suddenly the boat appears right next to a city when we see the minions flying past the ship. (00:05:15)