Other mistake: In the scene in which McClane and Farrell are in the car on the way to the Eastern Hub, the shots alternate between looking at McClane then looking at Farrell. When the stars are in profile there is visible through the subject's car window the landscape passing by, but the same hill appears outside each window. (01:00:55)
Other mistake: After the scene where the building explodes when the gas was rerouted to the station, there will be a shot of the main bad guy staring at the computer screen saying the connection failed. On one of the two connection lost screens, it says, "connection faild", this is not how you spell "failed". (01:16:45)
Other mistake: At the end Farrell is in the ambulance and talking to McClane. When doing so his hair is perfectly clean. After being through what he and McClane have been through in the last 24 hours, it's extremely unlikely his hair would be that clean. (01:58:35)
Other mistake: Where does the fire in the gas pipes come from? Even though a lot of gas - and thereby pressure - is directed towards the power station, why do they blow up under ground far away from the power station?
Other mistake: When Gabriel turns off the lights in the tunnel, we see a shot of cars heading towards the police car as the lights turn off, and not one of them turns its headlights on, or has their lights on already. With that many cars in the tunnel, there'd be at least one that was fitted with auto-lights, let alone people who put their lights on by choice in the fairly gloomy tunnel.
Other mistake: When Gabriel looks at McClane's police record it mentions McClane receiving a commendation for the Chicago airport incident in 1990. That incident happened at Dulles Airport in Washington D.C., not Chicago.
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