The Origin of the Fantastic Four: Part 1 - S1-E1
Other mistake: When Puppet Master is making a clay figurine of Thing, the mold he uses doesn't have a Thing-shaped interior with which to make a detailed figurine. The figurine should have just looked like an undefined human-shaped figure. (00:15:20)
Other mistake: When Tony Stark is suiting up to rescue War Machine, the device he wears on his chest to keep the shrapnel out of his heart is missing. (00:01:52)
Other mistake: Despite being Shield agents the guys shown in the target practice room wear the same uniforms as the guards working for The Kingpin.