Other mistake: At the end of the movie, when it was 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.
Plot hole: When they are trying to find a way to stop the lava from going in to the museum, Tommy Lee Jones has an idea to push a bus up against the museum to divert the lava. The plan works but the lava does not melt the bus like it melted Tommy Lee Jones' car or the fire truck. Even though the bus melts later in the movie, it doesn't melt it instantly like it did with the fire truck.
Trivia: The lava was primarily made of methylcellulose, the thickening agent used in fast-food milkshakes, and the ash was made mostly of ground newspaper.
Trivia: The story about the Mexican man working in his field in 1943 when a volcano rose under his feet is true. It is the Paricutin Volcano in Mexico. It rose over 1,000 feet in a week and is one of the seven natural wonders of the world. http://wonderclub.com/WorldWonders/ParicutinHistory.html.