When Matt Trakker is first seen sitting behind a desk in his mansion, his body is too large in proportion to the desk. In the next shot shown from overhead the proportions have been fixed.
Tiny Rhino fires it's lasers at Vanessa, managing to shoot over her shoulder in the first shot, but hitting her feet as well as Mayhem's in the next shot. The animation cel of the lasers should have projected under Vanessa instead of over her during the first shot.
The drainage grating is removed by a mysterious pair of hands. When the hands move it to one side, the arms stay attached to the grating instead of letting go of it. Later, when the cars are pulled into the grating with a magnet, the hole is much smaller than before.
When the vehicles of M.A.S.K. burst into V.EN.O.M.'s hideout, four villains mouth the words 'It's M.A.S.K.' but only one voice is heard.
Mayhem's lips don't move when he says "But we're not, get them! I'm gonna love breaking in these new shoes"