Deliberate mistake: When Johnny is watching the news footage of the Hulk battling Thing, all the footage is merely clips from that portion of the episode, including parts of the battle that were happening before the camera crews even showed up. (00:02:50 - 00:07:20)
Continuity mistake: When Sue is talking about her father going into hiding, the camera is on the right side of a photo album, showing a newspaper clipping of him being wanted for murder. When the shot changes, the contents of that page are now three different photographs. (00:07:32)
Continuity mistake: This series is in the same continuity as the other Marvel animated series that aired around the same time - X-Men, Spider-Man, Iron Man, and The Incredible Hulk - by virtue of each aforementioned show crossing over with at least one of the others. This episode, which originally aired on November 4, 1995, features a cameo appearance by She-Hulk. However, Jennifer Walter's didn't become She-Hulk until the season 1 episode of The Incredible Hulk entitled "Doomed," which originally aired on November 3, 1996. We know "Doomed" takes place after this episode, because the later episode of this series, entitled Nightmare in Green, features a plot where Dr. Doom meets Hulk for the first time after discovering that he is also Bruce Banner, and attempts to use him to destroy the Fantastic Four, and "Doomed" showcases Dr. Doom once again trying to use the Hulk to suit his own ends.
Plot hole: When Frankie tries to get Johnny to chase her out into space, he tells her he can't follow and warns her to turn back due to the thin atmosphere not being able to keep their flame powers active, and indeed her flames die out, and she begins falling back to Earth. However, several minutes later in the episode, both of them are just outside of the Earth when Terrax arrives, and again at the end after she becomes the new herald of Galactus, Johnny follows her into space well beyond Earth before his flames die out. (00:03:15 - 00:06:03)
Other mistake: This series is in the same continuity as the other Marvel animated series that aired around the same time - X-Men, Spider-Man, Iron Man, and The Incredible Hulk - by virtue of each aforementioned show crossing over with at least one of the others. However, early in this episode, the Fantastic Four receive help from the Avengers, and two of their members are Iron Man and She-Hulk. Iron Man is not a member of the Avengers in this universe, he is a member of Force Works. She-Hulk is not a member of the Avengers either. She is an attorney and is also frequently off doing missions with Bruce Banner/Hulk. (00:01:52)
Continuity mistake: When Dr. Doom defeats Thing, he leaves him in an immobile state, telling him he won't be able to move or speak for the rest of his life. When Johnny and Crystal find Thing moments later, his fingers are in a different position than they were when he was first turned into a living statue. (00:07:05 - 00:08:00)
Continuity mistake: When the heroes regroup by the Fantasticar after regaining their powers and taking out a few Skrulls, from the top going clockwise, it is Thing, Sue, Johnny, and Reed, and they are surrounded by Skrulls from all sides, with Morrat standing directly across from Johnny. A couple of shots later, from top going clockwise it is Johnny, Sue, Reed and Thing, and Morrat is now standing directly across from Sue. Additionally, the heroes are only now surrounded on their left and right, but not in front. (00:19:41)