Continuity mistake: In the beginning of the movie, after "mentalboy" has filmed Lester and Janey in the kitchen, Lester looks out the window. The kitchen is lit up, and it is very dark outside. This would naturally produce a reflection in the window, making looking outside impossible. There is no reflection. (00:10:00)
Continuity mistake: When Carolyn calls Lester and Jane out whilst honking the horn, the shot changes from her to the house several times. Each time the shot is facing the house the side mirror is flapped in and while facing her it isn't. Additionally, in the same scene when Lester comes out, his briefcase opens and his cellphone falls out toward the car out of reach, when he picks it up again it's right next to him.