Near the middle of the movie, when Lucy is sneaking into Summer's house to steal their dad's key card, it is night outside. When she walks in the foyer, the glass windows on the front door show that it's daytime and even show a pedestrian walking down the street.
At the end of the movie when the killer is stabbed, a close-up shows the knife has not stabbed him but is somehow back in his chest in the next shot.
When Summer holds a newspaper, in the close-up she has the sweater, hands, and nail polish of Lucy.
When Lucy first travels back in time and talks to Mr. Fleming about time travel, Quinn places three beakers of liquid on top of a bench, knocking over a glass at the same time. As he begins to walk around the bench, there is only one beaker of liquid on top. When he's standing in front of the bench, there are three beakers again.
When Lucy and her mother leave Summer's room, the music's stopped, but no one turned off the CD player.
When Lucy first enters Summer's room, she opens and looks in the CD compartment, but then presses the play button for the cassette tape.
2G was the standard for cell phones in 2003 and, while it isn't used in the U.S. any longer, modern phones could still connect to it because it is used in other countries. Lucy's phone would have gotten a signal, but no service, since it wouldn't have been activated back then.
At 22:50, after preventing Quinn from getting thrown into the river, Lucy says, "the video would have been viral on Twitter." Twitter was renamed X in July 2023, so there is no Twitter in 2024. Howewer, most people still call it Twitter, or intentionally ignore the name change.
Summer uses Kopari lip gloss in 2003, but the brand didn't launch until 2015.
The song "So Yesterday" by Hilary Duff included on the soundtrack was released some months after the events of the movie took place.
Quinn has numerous Dungeons & Dragons books on the shelves in his room, and among them is a 4th edition Dungeons & Dragons book. The movie is set to take place in 2003, but the 4th edition of the Dungeons & Dragons rules weren't released before June of 2008.
When Lucy and Quinn are inside SONR labs, British electrical sockets are visible despite the film being set in the U.S.
When Lucy and Quinn walk past a woman with an umbrella, she's seen sitting next to a Manitoba Hydro sign, although the film is set in the U.S.
An adult Quinn builds a time machine so he can go back in time to kill Summer and her friends, even though in his and Lucy's time-line Summer and her friends had already been killed 20 years earlier, so he has no reason to plan murders that have already happened.
Quinn looks at the smart phone like it's some unfathomable future technology, exclaiming about it being a pocket computer. Pocket PCs were readily available in 2003 from brands such as Hewlett Packard, XDA, HTC, running many different operating systems. Some devices had cameras and cell network capabilities.