Other mistake: Walter Paisley is cleaning the floor and encounters the first robot. The whole reason the robots are patrolling at this point is that the shopping centre is supposedly closed to customers, as it is nighttime. However in this scene you can see quite a few people shopping on the upper tier of the mall above Paisley.
Factual error: When the first robot is trapped in the lift, the teenagers shoot the propane tanks on top of the lift in order to get them to catch fire and explode, killing the robot. However the teens are firing pistols or shotguns, and these do not contain incendiary ammunition, which is what is required to make the propane to catch fire.