Revealing mistake: After Dante's Peak explodes for the final time, the pyroclastic cloud rolls down the mountain toward the town. Harry shouts "Don't look back, kids, don't look back." and then we are shown a wide shot of the truck zooming down the ash-choked streets. Unfortunately, it's painfully obvious that this is either a miniature or CGI shot, because the truck isn't leaving any tracks behind it or trailing any clouds of ash despite its high speed.
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?
Plot hole: Afterward the pyroclastic explosion Rachel, Harry and the kids get into the mine. There are a lot of things of Rachel's son in there: Sodas, chips, tapes and a tape player/recorder among them. There is a shot in which we can see a pile of tapes on the player, in almost perfect order. But if we consider that the whole town collapsed because of the earthquake occurred before the eruption, then those tapes must have fallen down, surely?
Continuity mistake: When they are driving up the mountain to get to her kids, they cross a river and they get hit on the passenger side of the truck shattering the window, but later when they show them going up the mountain there is a new window there.
Continuity mistake: Mr. Custer has an on and off problem with his glasses when registering Harry Dalton. When seen full face, his glasses are in his shirt pocket--yet, when seen from the back, he has his glasses on!
Other mistake: The first time Rachel brings the team coffee, she has six coffees on a tray. There's five cappuccinos and one regular coffee for Harry. On the tray there is five coffees on one side and one coffee at the other side, obviously being Harry's regular coffee, so that Rachel would be able to tell the them apart. But when she gives Harry a coffee, she gives him one of the five standing together, so he obviously got one of the cappuccinos.
Factual error: In the scene where Pierce Brosnan hot wires the pickup truck, it is clear the truck has a locking steering wheel and gearshift. Even if he connected the correct wires to start the truck, he wouldn't be able to move or steer it.
Plot hole: When Harry and Terry are on the volcano with Spider Legs and Terry breaks his leg, they need to call for a helicopter to come rescue them. How did they get up there? I don't think they were able to hike up the volcano, let alone hike up there with Spider Legs and their other equipment. After all, it took 4 people just to get Spider Legs off the rolling cart in the motel parking lot.
Revealing mistake: In the scene where the van is about to be flipped over on the washed out bridge, wires can be seen on the hood of the miniature that pulled it across.
Factual error: In the scene where Paul and the National Gurardsman are leaving the motel for the final time after packing up, Paul follows the two Humvee's out of town. Being a Guardsman myself, I know for a fact that all civilians would be in front and Humvees would be following them. It is standard operational procedure.
Other mistake: When the volcano begins to erupt, cars are seem driving out of the city. When the motorway collapses, and the cars go with it, in the close up shots, there are no drivers inside them.
Continuity mistake: Just after the helicopter crashes, check out the two full-view shots of the vehicle that show the top running lights. Of the three that are left, the middle one has been knocked crooked. After these two shots, the lights are all perfectly aligned again.
Continuity mistake: When Rachel and Harry are crossing the river in the truck, they get obviously very wet. By the time they get to Ruth's lodge, they seem quite dried out.
Deliberate mistake: In the shot right after Harry successfully maneuvers out of the river, the truck he's driving should still have water pouring out of it. When he was initially pulling out, the water in the truck was still up to the bottom of the steering wheel, and we can see it draining out, but it would take much longer than a second or two for the water to fully drain out- the truck's completely dry and not leaking at all.
Deliberate mistake: Geologically, the type of volcano in the movie shouldn't erupt so much low-viscous magma; but due to make the volcano exciting to the audience, the movie makers probably deliberately ignored this mistake.
Continuity mistake: Near the start of the movie, during the violent eruption when the major characters have quickly loaded their gear into the truck and are trying to escape, the shot changes back and forth from looking into the cabin of the truck from the front and then looking forward out through the windscreen. The windscreen wipers are running at very different speeds. From inside, they run very fast (probably so you can see out through the mud) and very slowly from outside (probably to accentuate the mud etc).
Plot hole: After the volcanic eruption, there's a scene where we see our heroes going across a lake, which has been turned to acid by the lava. If this acid is strong enough to corrode the propeller, then surely the fumes would be destroying their lungs, in which case they should be dead, or close to it.