Plot hole: When Kate went to the aid of the man lying in the street, she was very upset about his death. Would she not have got his name being that she works in a hospital? Also she could have easily Googled him in 2006 to find out where he was or that he had died already. It seemed an obvious ending that she would save his life to change history.
Plot hole: At the end of the movie, when the water drains away, there would be no way for a) the amount of water to drain away that did and b) for the mammoths that came through to be completely dry. (01:16:20)
Plot hole: When Farley is preparing to rescue Simon by towing him up with the rope attached to his van, he drops his "cigarette" into his crotch, causing him to floor the gas pedal. His back wheels begin to spin. This is impossible because the vehicle is still in Park. There is even a camera shot of the "PRNDL" proving this. I guess this goof is employed by the filmmaker to increase the dramatic effect of him subsequently slamming the Dodge into drive a second later and careening through the fence. (01:07:35)
Plot hole: When Melvin receives the phone call from a lost Diana, he is staring at his goldfish, swimming peacefully in the bowl in his apartment. He unsuccessfully attempts to rescue Diana, then goes to the night club where he encounters an inebriated Ginger. He escorts her home, rebuffs her advances, and goes to his own apartment. The next morning, he goes to her job site, where, inexplicably, the goldfish has now taken up residence. Unless Ginger's flunky somehow broke into Mel's apartment and absconded with the fish, there is no way the fish could have gotten there on its own.
Plot hole: Viola convinces her mother she is staying at her father's house for two weeks, but who covers for Viola with school administrators? As far as we see in the movie, Viola is not reported absent from either Cornwall or Illyria (a transfer would explain not being reported absent from Cornwall). Administrators or class mates at Cornwall would find it strange that she stopped showing up, since soccer training had already started. It is unlikely Viola officially transferred to Illyria, since her parents would have to sign off on that, or at least start paying her tuition to a different school. Viola could have called in sick to either Cornwall or Illyria, but this is never addressed in the movie. And if the soccer program at either school started before the official school year, it would make no sense for them to have a biology class with students not enrolled in athletic programs at Illyria.
Plot hole: Just before the bomb explodes at the beginning of the movie, the guard is looking for the car playing 'don't worry baby' on it's stereo. In the final timeline the disaster has been averted, the bomb having gone off, and the ferry docked. After this, when Carlin is in the car, the same song is playing, but the bomb was set to a timer.
Suggested correction: That's why the movie is called Deja Vu and thats why Carlin gives the girl such a strange look. The song triggers a feeling of Deja Vu for him and it almost feels to him that he knows her. Besides, a different station could have been playing the song.
Suggested correction: The first time it plays comes off the radio. The second time we hear it mid-play. It could have come from a different station, the same station could have played it again later, or a CD or cassette being played in the car.
Plot hole: The contest happens on July 16th 2005. Karen appears on a magazine cover as the successful new manager in the September 2005 issue, which is really a bit too fast considering in barely a month Bowker's wife would have had to divorce him, sell the company and reduce him as the homeless bum shown, on top of Karen actually taking over.