During the scene where Tommy is grinding a part of a brake or something, the piece shoots out of his hand. After bouncing all around the workshop, it goes through the window. If you look very closely, you can see the squibs, or small explosives used to blow the hole in the glass. They are kind of hard to see because the camera isn't focused on the window. But you should be able to see 3 small blacks dots, those are the squibs. Did a brake part fly through that window, or are they experimenting with explosives in there? This is a slip-up only movie geeks would notice. It's really not even worth looking for.