Continuity mistake: As the detectives are talking to their captain about the bank robbery, in a shot from behind the captain he takes his hands off his food and clasps them in front of him. The shot cuts to his front as he starts talking, and he moves his hands and clasps them again.
Continuity mistake: In episode 2.09 of related show "Arrow," during the day the particle accelerator is activated, Barry spends his time with Oliver, Felicity, and Diggle in Starling City. He arrives back in Central City just in time to see the particle accelerator activated, and he is in his lab because he was too late and missed the line at S.T.A.R. Labs. In episode 101 of "The Flash," Barry was never in Starling City during the day the particle accelerator was activated, and he and Iris actually attended the event. (00:02:10 - 00:11:30)
Revealing mistake: When Iris is sitting in bed recording the message for Barry, she is holding her iPhone so that the start button is on the left side, but the "video record" button appears on the right. Additionally, her right hand covers the speaker area of the phone fully, including the front facing camera. It is impossible for Iris to be recording the video live and reveals that the video footage was recorded ahead of time. The illusion that the video is being recorded by Iris live is further ruined by the fact that the room lighting is turned off in the scene, but the recording is well lit. (00:02:55)
Deliberate mistake: Early on, The Flash's immense speed caused his clothes, and those of people he was carrying, to catch fire because of the air resistance. Gave him an excuse for the suit, but this effect was fairly quickly forgotten, with him frequently carrying civilians or running out of costume with no ill effects at all.
Continuity mistake: As Barry is going up the stairs in the police station and says he doesn't want a museum dedicated to him, a man and a woman are walking down the stairs. However, a second later, the shot has changed and the man is now a different person and the woman has gone completely.
Plot hole: During the fight at sundown, Cold gets a hit on Flash and he goes down. Eddie takes a shield and goes to help. But Flash was in the middle of the street with no obstacles around, and it's implausible that Eddie could have gotten to him without Cold or Heat seeing him.
Other mistake: When Cisco and Cailtin call Barry to tell him about the man with the gun, Cailtin says he has a gun. Cut to the cops pursuing him, the perp shoots at the cops, and one of them seems surprised that he has a gun. There's no way Caitlin could have known about the gun without the cop knowing (which would presumably have been relayed over police communications).
Continuity mistake: Iris is backing up away from her evil counterpart while holding her hands up in front of her. She has fake white nails on, but in the next shot with evil Iris in front view, it shows her hands and there are no fake nails and no nail polish. These are the hands of a stunt double backing up. (00:21:28)
Continuity mistake: When Cisco opens up a portal, Caitlin and Dominic are next to it and seen moving towards it, with Ralph standing in front of the group (as if he's covering them). In the next shot, Caitlin and Dominic are going through the portal followed by Cisco, but Ralph is nowhere to be seen as he's already entered the portal. There should be no way for him to completely enter the portal first.
Continuity mistake: The explanation that Cisco draws about the endless cycle changes in between shots, not counting the added words "past", present", and "future" (which he could have written when off camera, not that it would make sense to do so since he already explained what was past, present, and future). Where the loop crosses itself is higher up as well as the "FF + TR" being different.