Continuity mistake: During the plane and semi scene the front of the semi gets shot to pieces. If you look at the windscreen when the semi comes back onto its wheels it's more like a protection cage, then in the next shot it's gone.
Continuity mistake: The wounds on John's right eyebrow and cheekbone change size, disappear, and switch from fresh to dry then back again throughout the film.
Continuity mistake: When John drives over the fire hydrant and the water jet hits the helicopter, the gunman falls out and ends up on the ground between the car and water jet. When John drives off in the long shot his body has disappeared.
Continuity mistake: When Detective Maclean is trying to convince Matt to go with him, Matt tries to open a window, so he can escape, but the window won't open. Later, after all the gunfire, both men escape through the now-working window.
Chosen answer: Looks like a standard 5 mph speed sign to me. You can't read too much into that. In addition though, Bruce Willis has already stated in a few interviews that he'd be more than happy to do another one.
XIII