In the episode titled "Rush", when Thundercracker assaults the Autobots' base, not only does he speak erroneously in Starscream's voice, he also refers to himself as Starscream. This goof was fixed for the DVD release.
Crosswise is erroneously referred to as Smokescreen for a while after his debut. He is later called by his correct name. Later reruns of these episodes fixed this error, although foreign dubs have still been produced based on the incorrect dialogue.
The goof item below may give away important plot points.
As his body evaporates, Galvatron mouths his dying words, but only a grunt can be heard. In the original version, he is supposed to say Optimus Prime's name, but the American version neglected to dub this line.