Factual error: Considering Sigrit and Lars are similar ages in the opening flashback to 1974, 41 year old Rachel McAdams should be a similar age to the 52 year old Will Ferrell, which she clearly isn't. Alternatively, if she's meant to be in her 50s, there's no way they'd have been able to have a baby together.
Factual error: When the Americans are driving Lars to the contest, after spinning out, the driver puts the car in second gear (pulls the shifter back) instead of first and then speeds away from a standstill. Second gear would not sound like it did in the movie plus the acceleration would have been very weak, possibly stalling.
Continuity mistake: When Will Ferrell and Pierce Brosnan have their conversation about the house in front of the tomb, the natural light changes very between shots (when Brosnan says "I make my money fishing" Ferrell's shoulder and face are fully lit by direct sunlight, gone at the next cut). (00:06:30)Sammo
Continuity mistake: During Abba's performance, the toy young Lars is playing with changes position between shots. Specifically, when the song says "At Waterloo Napoleon did surrender", the wooden tile laid flat on the step is pointed towards the camera, and after the cut it moved 90°. (00:01:35)Sammo
Continuity mistake: When the flaming severed arm falls in front of Lars and Sigrit, its position is different between shots; it fell across the horizontal lines of the railing's shadow, but in close-up it's parallel to them, and when they kick it, it rolls over obviously different spots (easy to tell by the width of the shadow). (00:26:45)Sammo
Continuity mistake: During the exhibition of "21st Century Viking", one of the bar patrons calls the attention of the others; his hand changes position on the glass between shots, the guy with a shaved head and short black beard is holding his beer with his right hand instead of both, etc. (00:21:45)Sammo
Continuity mistake: Neils Brongus puts next to him in the conference room the DVD with Demi Lovato's performance. The DVD is placed in the first shot on top of the remote control, but already in the next shot it's off it, and it keeps bouncing between these two positions and different ones depending from the camera angles. (00:13:20)Sammo
Character mistake: When the camera pans to the frame containing the picture of Lars with his departed mother, the caption says "In memory of our beloved mother Mirra Gunnarsdottir", with a plain O. On the cross, her name is Mirra Gunnarsdóttir. It's two different letters, and it'd be pretty strange to honor the memory of your mother printing a photo with her name in the wrong form. (00:01:10)Sammo