Go to the part when they use the pinch to knock the power out and when they show "the strip" (a.k.a the main street) all the electricity goes out but so do the lights on the cars! WHOOPS!
The device was used to temporarily knock out electrical power. Yes, the cars' headlights went out because they are powered by - ELECTRICITY! This is not a slipup - your car's headlights are powered by the battery (which produces ELECTRICITY)!
This slip up, isn't really a slip up at all. "The Pinch" the guy sets off in the parking lot is an EMP. Which, if you listened to the movie, you'd know takes out all electric devices. Including battery operated things. Such as cars.
A "pinch" kills any, and all, electrical currents. Everything has some electicity, even people, you can feel a pinch, so the cars goin out would happen.
EMPs (Electromagnetic Pulses) will take out anything that runs on electricity. Toys, PDA's, light bulbs, and headlights - they can all be downed by an EMP.
In the movie, they describe the pinch as taking out a high broadband electric circuit. So the car's headlights weren't supposed to go out. Even if they did, a battery operated device wouldn't be able to recover, such as the remote detonator which George Clooney uses.
this is not a slip-up, the pinch disrupts all electrical items, which includes car alternators, needed to produce electricity to create spark for the spark plugs, which are needed to detonate the fuel and run lights, and all other electrical functions, therefore no slip up