Factual error: Several episodes depict the moon in the night sky as being far too large.
And the Sea Shall Give Up Its Dead - S1-E1
Other mistake: When Tony Stark is typing on the keyboard, some of the keys are depressing without him touching them.
The Origin of the Fantastic Four: Part 1 - S1-E1
Continuity mistake: When Sue turns invisible for the first time, Ben Grimm's hair alternates between being gray and auburn. (00:07:46 - 00:09:08)