Other mistake: When Elastigirl is chasing the high-speed train, as she is in the middle of making her second final jump to reach the train, if one were to look closely, the train actually drives right past her before she lands. (00:31:40)
Trivia: The film "Dementia A-113" on the theater is a Pixarified reference to the Francis Ford Coppola film "Dementia 13".
Continuity mistake: After the old lady thanks Dash for saving her from getting crushed by the flung blue vehicle, that vehicle suddenly has been moved slightly to the left from its original landing spot. Also, another wrecked light blue vehicle was visible as the aforementioned blue vehicle hit the ground, after the old lady thanks Dash, it has suddenly been replaced by a white wrecked vehicle instead. (00:05:45)
Answer: What happened to Syndrome happened high in the air, Elastigirl mentions she can't see whats going on and panics. They never actually see what Jack-Jack is doing and think Syndrome is hurting him.
lionhead