Revealing mistake: When the Operative is reviewing Mal's file, Mal's first name is initially spelled as "Malcom", in the next shot that has been corrected to "Malcolm".
Revealing mistake: Ryan Reynolds is wearing red-eye contacts. You can see the whites of his eyes around the red circle of the contacts in several scenes - most noticeably, right after the home movie he's watching down in the basement melts.
Suggested correction: Not a mistake. Ryan Reynolds actually had bloodshot eyes from being awake for nearly two days of filming. He stated it in an interview for the movie.
Continuity mistake: In the kitchen scene at Greg Kinnear's house when Pierce is convincing Greg to fly to Arizona with him to do a job, the clock on the wall behind them first reads 5:45 am. In the next shot, the clock reads 5:28 am.
Continuity mistake: When Ice Cube is running out from jail, he kicks out the ventilator and climbs up the roof. Next shot from far, the ventilator is in its place. (00:11:10)
Continuity mistake: When Sandy and Tessa are talking in Sandy's office, the cricket ball on his desk vanishes, then reappears in a different location. (00:39:10 - 00:39:40)
Visible crew/equipment: In the final scene where Evey activates the train full of explosive devices, making it head towards Westminster Palace, a camera mounted on a crane is reflected on the train glass. (02:02:16)