Factual error: The entire time the volcano is shown erupting, the volcanic lava is instantly melting everything it touches despite the fact that in real life volcanic lava would not have any human being melting because lava is more dense than water. Also lava in real life cannot instantly melt things. And no object should be sinking if caught in a lava flow.
Factual error: At the end of the movie, when it's raining, with all of the smoke and ash in the air, the rain would have likely been very acidic. The people probably wouldn't be enjoying the sudden downpour.
Factual error: In the scene during the evacuation of the Cedar Hospital, Dr Calder is assessing a patient and uses the defibrillator on him. She puts the paddles in the wrong spots on the chest - right hand one over the abdomen and the other over the left lower chest. As a doctor she should know to put the right hand paddle over the upper right chest. This is important for the normal electrical pathway through the heart to reset and restart normal cardiac activity. (01:27:22)
Factual error: When lava engulfs the streets, aluminum light poles do not melt (it has a melting point of 1,212° Fahrenheit), yet entire vehicles with steel frames melt into the flow (steel melts at 2,750° Fahrenheit). Lava reaches around 2,000°.
Factual error: The volcanic ash that can be seen throughout the film is too big to be real. The ash shown is something like 1-2 cm in diameter. Volcanic ash is not that big in real life; it is not more than 2 millimetres in diameter. (01:02:30)
Factual error: It is impossible for a volcano to form under Los Angeles. The San Andreas Fault traveling through the west coast is a "Transform" fault in which the Pacific plate and the North American plate creates a fault line that only slide past each other. In order for a volcano to form, two plates need to subduct. Because there is no ripping apart or subduction taking place along a transform fault, there isn't any magma formation to lead to volcanoes.
Factual error: Mount Wilshire is basically a shield volcano in the form of a tall stratovolcano because shield volcanoes only shoot ashes, gas, and lava (and pyroclastics for pyroclastic shield volcanoes). The lava itself behaves as if it was sentient. During the lava geyser scene, small particles of molten lava seem to be falling directly on people despite the fact that no-one was anywhere near it except for Mike, his daughter, and the little boy.