Continuity mistake: When the bad guys are clearing the mall, the camera follows "Rudolph" as he clears an exit on his skateboard. As he starts to clear the exit a young man with a black jumper with a white top half can be seen on his feet moving away. In the next shot he is sitting down with a bag on a bench and then gets up to leave.
Other mistake: When Blart and the SWAT guy are pursuing the bad guy, they have a blue police light on top of the car (the magnetic kind). Interior views of the car show blue flashes on their faces, which is impossible with the light being on the roof. The exterior shots confirm that the inside of the car is completely dark.
Other mistake: In the beginning of the movie, Paul is at the NJ State Police training grounds. The training grounds, while near the ocean, do not have any islands just off the coast, nor is the ocean water nearby. The police cars used are painted light/metallic gray Massachusetts State Police colors with a blue stripe down the side. NJ State Police cars are white with red or red/blue lightbars with a blue & gold triangle logo on the front doors.
Other mistake: It is November, since Blart is worried about Black Friday. The kid's clothing store they are seen walking by toward the beginning of the film has Summer clothing and bathing suits in the window. I used to to work in retail and while stores were far ahead of the weather, they are never that far ahead.