Factual error: When Christian Slater and Minnie Driver are trying to get out of the water before it reaches the transformer, there is a real big problem with this scene. If there really was a flood and it was rising that high, the city should have shut off the electricity to the entire city, to prevent any electric hazards that may happen in that situation.
Continuity mistake: When Christian Slater is in jail, he wakes up to the sound of water. He than swings his legs into the water and stands up. If you look, his pant legs are already wet before he swings his legs off the cot.
Character mistake: Just after the shootout between the robbers and the armored truck drivers at the dip in the road, the teacher that is with the robbers pumps the shotgun about 3 times, without firing a shot after they crossed the water. This would just waste a perfectly good cartridge, since they hadn't been shot yet.
Factual error: In the scene inside Karen's house, the water level rises way too fast. The house is not in an isolated pool of water, but connected to the entire city. So for the water level to rise several meters inside the house would mean that the water rose several meters in the entire city. And that could not happen that fast. If it was a wave from the lake, the water level would drop again.