Factual error: Upon hearing that the bombs will drop in 3 hours time on Santa Monica, one marine sets the countdown timer on his watch for 3 hours and starts it. The watch is a Casio G-shock GW-9000 Mudman. This particular watch only has a 1-hour countdown timer. (00:24:15)
Factual error: They were hot wiring the city bus - that make of bus does not have a key to start it. There is a power knob you turn to run, and then push a start button.
Continuity mistake: During the final shootout, there is a shot of Nantz firing his handgun. A few shots later, Nantz pulls out his handgun and begins firing.
Audio problem: Near the start of the movie, the Marines are out training and are approached by a group of Hum Vees that are to take them away because of the impending battle. The engine noise from the Hum Vees however is totally wrong, and is distinctively from a Willys Jeep.
Continuity mistake: Towards the end after the control drone has been destroyed, Nantz is seen advancing against the Aliens with his hand gun. The next shot he is seen changing from his rifle to his hand gun. The next shot his hand gun is seen to be empty (slide locked back), but the next shot his handgun is loaded again.
Continuity mistake: When Lieutenant Martinez is ordering Staff Sergeant Nantz to evacuate the troops and civilians off the freeway, there's all this black ash type stuff all over his mouth and even in his teeth. Afterwards when Nantz runs off, LT Martinez says his final speech on the radio but you can see his mouth and teeth are clean as. Also the sun is shining on his face and disappearing again throughout this scene. (01:06:50)
Factual error: In the scene where the marines are being told what their mission is and where to go, they are told to go to the police station at Venice Blvd. and Amoroso - two streets that don't cross. Later, when they are on the freeway heading toward the FBO, they find out that the offramp is blown. They are told to use the Robertson exit, even though they are on a time crunch. The Robertson exit off the I 10 is about 3 exits from the exit they missed.
Other mistake: Between the freeway battle scene and the convenience store scene; there's a transition shot where you can see the characters walking through a bombed out LA with the alien aircraft flying above. If you pay attention to them, instead of following the camera, at the end of the shot you will see them all stop moving mid-stride while everything else in the shot continues to move.
Answer: Most likely, coastal cities or cities near a large body of water.