Revealing mistake: (Part 4) After Omega takes off his mask and looks in the mirror, when he sees that his physical body doesn't exist he puts his mask back on and screams, but when he shouts, "If I exist only by my will, then my will is to destroy," Omega's mask pops up off his face, and we can see the actor's blackened face as he shouts the last bit of his dialogue. (00:07:10)
Continuity mistake: In flashback, we see the one-armed man kill Helen Kimble by striking her with a heavy lamp. All through the earlier seasons, though, we were repeatedly told that Helen had been strangled. (00:28:50)
Deliberate mistake: During the 6/11/04 episode, Jax and Courtney are having dinner in Italy. Carly, who is in New York, does some sleuthing, finds out where Jax and Courtney are, and flies to Italy. Miraculously, Carly manges to fly from New York to Italy and arrive while Jax and Courtney are still eating dinner.
Continuity mistake: When the lab equipment shorts out after the earthquake, the unconscious Major Brothers is lying with his head propped up against the wall. But when Hamilton reaches him in the next shot, he's suddenly lying flat and is nowhere near the wall. (00:21:45)
Who Killed Avery Lord? - S1-E23
Visible crew/equipment: After the woman in the negligeé goes back into her apartment, a crew member's shadow appears on the corridor wall. (00:42:00)