Continuity mistake: When Harry, Rachel, and the kids drive into the mine at the end of the movie, there are already tire tracks there before them, despite plenty of ash having fallen to cover tracks made earlier. Who would have been going in or out of the mine before that?
Revealing mistake: When the lava comes through Ruth's house you can see that it's done with CG. The reddish "streaks" look like from a sci-fi movie. The shapes of the lights and streaks are unnatural.
Continuity mistake: When Harry and Rachel are making their way back to Rachel's house after the volcano's initial explosion we see them go down an alley to avoid all the traffic. As they move down the alley we see a pile of bricks fall from a roof and hit the front of the truck and window which causes large cracks in the glass on the passenger side. However, immediately after this we see the car and in all shots from the outside of the vehicle the cracks are no longer visible.
Continuity mistake: The watch of one of the councilmen shows 1.40. (the stubby one in the white shirt, the one that tries to get the investor to invest in the city). Some 18 seconds later it shows 1.45.
Factual error: When being rescued from the crater, Harry tells the helicopter that he is 600 feet below the rim. But earlier, when first the other scientist and then Harry went down into the crater it took a very short time - that could not have been 600 feet. And also from the look of the distance it was more like 150 feet. (00:13:10)
Other mistake: During the end-credits, someone's title is listed as 'Mr. Bronson's Driver' instead of Brosnan.
Other mistake: When Harry gets out of the truck at the motel to retrieve E.L.F., there appears to be a blue & white storefront sign in the background that is lit, when the rest of the town is dark due to no power. Either that, or it is a really bright, white, non-electric sign that has been completely unaffected by the ash that is coating everything else in town. (01:26:40)
Continuity mistake: When they get in the boat to cross the lake, they notice dead fish floating on top of the water. On the other side of the lake, there are no dead fish.
Factual error: Harry was able to drive across the lake as the truck he was in was fitted with a snorkel. The other cars that drove into the lake would have immediately stopped as the engines would have hydrolocked. Therefore the car that hit the truck wouldn't have been able to get there in the first place.
Other mistake: When the lahar slams into the bridge, it seems strange that none of Paul's windows shatter while he's trying to get across. A lot of mud and debris slams into all three vehicles, but their windshields and windows remain intact. Also, the bridge entrance is washed away, but the rest of the bridge remains standing, until it tilts upward and disappears, taking Paul and his van with it.
Continuity mistake: After Terry falls in the crater, watch Harry climbing down to him. Harry is almost down, then the scene flashes to him at the top of the rim again, and he makes his way down a second time.
Revealing mistake: When Lauren screams after seeing the bodies in the hot spring, someone grabs her, but it is none of the characters in the scene.
Character mistake: When Harry is registering, he has glasses hanging around his neck and is wearing another pair, presumably readers, while he checks in. After exchanging pleasantries with the proprietor, he takes off the glasses, puts them on the counter, and leaves without them. (00:08:05)