Secret Wars, Chapter 3: Doom - S5-E11
Other mistake: This series is in the same continuity as the other Marvel animated series from around the same time - X-Men, The Incredible Hulk, Iron Man, and Fantastic Four - by virtue of each aforementioned show crossing over with at least one of the others. However, in Fantastic Four, Dr. Doom had a German accent, and in this episode he has a Russian accent.
Suggested correction: It was a part of the character's redesign. The writer of Spider-Man, John Semper Jr, disliked the Fantastic Four cartoon to the point that he not only changed characters' designs but their voice actors as well.
Redesigning the character is not a valid correction, because it still creates a contradiction within the shared universe. Even under the pretense of redesigning the character due to not liking the previously established version, it would still be a deliberate mistake.
Phaneron ★