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.
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.
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.
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.
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.