At 53:45 of the movie, Paul and Maggie, in the Sheriff's car, driving to where the kids are in the river, turn off a dirt road to a black top road. At 56:41, Paul and Maggie, still driving to where the kids are, turn off the blacktop road onto the same dirt road, but from the opposite direction. At 1:01, Paul, now driving alone, turns off the dirt road yet again to the highway.
At 1:05:02, a deputy falls off a boat face down into the river. The immediate next shot is from the river level, and shows the deputy falling butt first into the water.
Despite being described as 'freezing', the pool that the piranhas were originally in had steam rising from it, indication that it was, in fact, heated.
When William Kat jumps into the water near the end of the movie..you can clearly see the markings on the swimming pool wall that they filmed the jump into.