When the Hobo is at the apartment getting beaten up, he gets his back stomped on with skate blades that cut his clothes and back up. In the following scenes, he is wearing the same clothes but the shirt is no longer cut, nor are there any injuries to his back.
When the bad guys approach the school bus, the rear exit door window clearly has no "PORTE DE SECOURS" sign on it (which is French for Emergency Exit). But just a moment later when the kids inside try to escape the burning bus, one of the kids can be seen at the rear exit door, where now can plainly be seen the inscription "PORTE DE SECOURS" on the window.
Where Slick is about to saw at Abby's neck, the blade on his saw is quite shiny and obviously real. In the next shot when he holds the blade to her neck, the blade is quite dull and flexes obviously, as it is made of rubber.