When Mr. Barker and Harry are walking past the warehouse, they step on a lot of broken glass, supposedly from the windows the vandals broke. However, since the rocks were thrown from the street, most of the broken glass would have ended up in the warehouse and the rest would have fallen just outside the building, not all the way out to the sidewalk.
Near the beginning when the night watchman is talking to the policeman, the officer says, "We'll pick him up" then starts to hang up the phone. As the phone is nearly in its cradle, he says "OK", but his lips don't move.