When Sam is attacked by the water demon, Prue does not get soaked and her hair is dry. However, when she kneels down beside Sam her hair is soaking!
When Leo "Orbs" with Piper up to the Lake, Piper needs to sit down and does so on a sun lounger. In the close-up shots of Piper/Phoebe/Leo, Phoebe is holding Piper's right hand. In the longer shots Piper has both hands on her knees. This continues back and forth until Prue talks to Phoebe.
Leo tells Piper that he couldn't heal her poison ivy because he can only heal when the elders allow him to. Later in the series this changes, and he can heal anything, even when the elders don't want him to.
When Jack Sheridan hangs up on Prue, a dial tone is heard although cell phone systems do not have one.
The little girl that portrays Prue in the flashback is the same actress that portrayed young Prue when they went back in time. However, when Patty died, it would have been at least 3 years later so Prue would have been older and bigger by then.
When the police officer talks to young Prue in the opening scene, he says he will take her back inside and carries her. But then he puts her down near the ambulance almost immediately after.
The ambulance in the opening flashback was a late 90's model. Ambulances of the's typically were of the station wagon/hearse variety seen in such shows as "Emergency" or even the movie "Ghostbuster" or the box style.
When the camp counselor is killed, there is a visible human stuntman leaping out of the water to get her.
When Prue and Phoebe drive up to the lake in her black Mazda Miata, a cameraman and boom mic are reflected in the driver's side door as they move up to get the shot of the girls inside the car.
When the girls come back from the lake/camp and wake up in the morning, they're all itching. They say it's like when Prue got poison ivy at camp when they were kids. Poison ivy doesn't grow in California, it's poison oak you have to watch for.
The morning after Sam used the powder on the girls, Phoebe's hair was down when Leo came to warn them about the campers going to the lake. However, when they are at the camp, she has a very fancy hair style.