Other mistake: When Greg is looking at the video of Rowley sitting on the foil ball, he notices it has only 4 views on YouTube. However, if you look closely, it has 24,963 comments - impossible if it's only been viewed 4 times.
Continuity mistake: At the end of the movie, Rodrick is driving Greg to school in his van. There is a close-up showing three flyers beautifully arranged on the top of the dashboard as they are driving along, and a soft toy. After Rodrick pulls away, there is no sign of the flyers and soft toy.timwain
Continuity mistake: In the scene where Susan's editors come round for supper, Greg and Rodrick have to return home by 9pm. And they do, as there is a lingering close-up of the wall clock in the sitting room showing the time as 20.55, and dialogue centring on this [i.e. the clock has not stopped]. There is then a lot of commotion about whether Susan's column is a wholly accurate depiction of the dynamics of the family that takes a good couple of minutes to play out. The editors make their excuses and leave, and we see the clock again as they are leaving, but in all the time this has taken the clock has not advanced. Then there is a further scene where the boys argue in the front room and after that has played out we see the clock again, and time has continued to stand still.timwain
Continuity mistake: After Rowley's magic show, Holly goes backstage to talk to Greg, but in the scene of Rodrick playing the drums, Holly is back in her seat in a matter of seconds.
Continuity mistake: Greg tosses the foil ball onto his bed and it lands near the center of it. However when Rowley accidentally sits on it, the ball is closer to the edge.