Revealing mistake: Why is there a full moon every night?
Revealing mistake: When Bullseye stabs Elektra with her own sai, watch the profile of the sai. If you look at the angle of the piece being held against her stomach and the angle of the piece that's coming out her back, there's an obvious bend, indicating two different halves.
Revealing mistake: When Elektra is fighting Daredevil on the rooftops, at one point he's on a different building, and she screams and jumps across the gap. There are a few problems, however. For one, she floats across the gap. The way she moves and how she stays in the air give away the fact that she's on wires. Another is that it is a very big gap between the two buildings. There's no way anyone could have made that jump normally, especially since the building she's jumping onto is roughly the same height as the one she's jumping from. Finally, the way she lands is impossible. She lands perfectly on her feet, without any stumbling, crouching, or having to slow down. Very sloppy stunt work.
Revealing mistake: When Matt's father has died and Matt touches his face, you can see that a vein is throbbing on the father's neck.
Revealing mistake: When Elektra and Matt are fighting in the playground, Matt goes to grab his staff/cane from the bench, but just as he is about to grab it his arm stops and a second later Elektra kicks his arm out of the way.
Revealing mistake: When Bullseye lifts Elektra and keeps her by the throat, you may notice her back - her outfit is not tight, as usual. The cause of it is the safety wires which help to lift her.
Revealing mistake: When Elektra stabs Daredevil, it changes to a shot from behind the glass. If you pause when the glass is intact, you can see a faint circle where the glass has been made to shatter. (01:11:46)