Two extras are shown carefully carrying a large pane of glass across a street. Cut to a chase. Cut back to the sheet of glass. Inevitably, a car is driven right through the glass, shattering it everywhere. The alternative is that a character walks into it, unaware that it's there. A standard gag is to have the extras see the first approaching car and scramble to get out of its way, only to have them relax and not notice the pursuer until he smashes through. See also Subverted Trope, where this trope is used as an example to illustrate the variations of it. Examples of Sheet of Glass include: