The slip-up is when the bad cops start attempting to scare the occupants of Precinct 13.

The way the lasers move, it is as if the pointers are right up against the windows. Impossible because all the bad cops set up a perimiter quite a distance away from building.

For the lasers to be that close to the windows, the cops would have had to take the pointers off of their guns and pressed them up against the windows.