Continuity mistake: When Mr. Fantastic is writing a problem on his chalkboard, right before the camera switches to Von Doom picking at his scar, you can see that Mr. Fantastic already wrote 4= Sxlo -4 and underlined it. When the camera goes back to Mr. Fantastic after the picking of the scar, he is now writing 4= Sxlo -4 and underlining it.
Picture edited by Super Grover