Veronica's hairstyle constantly changes, often between shots (bangs/no bangs).
Veronica picks up the cup of poison covered by its lid, but then in close-up she is carrying the relatively harmless uncovered cup, which a moment later is back on the counter for J.D. to look at.
Early in the movie, during the frat party, Veronica throws her burning glass out the window and it lands in a trash bin that sets on fire. When she leaves the house, the fiery trash bin is not under a window.
When Heather C is setting up her backswing with the croquet mallet, the positioning of Heather M behind her alternates between close and far back, depending on whether the shot shows their feet or upper bodies.
A police officer drops his baton, but has it back while chasing JD and Veronica.
In the cafeteria scene it's impossible for extras to pass behind or walk by J.D. since he is sitting in a corner by the windows.
Moby Dick does not contain the word "Eskimo" in that spelling, but does contain "Esquimaux" which is the French Canadian spelling.
When Martha Dumptruck is alone in the gym drinking her soda through a straw, we see through the bottom that she finishes it all. She then spills the finished soda on the front of her shirt.
When Veronica and JD are waking up in the school parking lot, Veronica holds her left arm carefully. When she takes the lighter and puts it on the palm of her left hand, it is obvious that this is not a real hand.
When Veronica comes out of the Snappy Snack Shack with the large Slushee that JD just bought for her, she waves it around and holds it with a bent wrist in a way she couldn't possibly do if it were almost full of a heavy liquid (revealing that the cup is actually empty).
While JD is chasing Kurt, Veronica reaches down and presses on Ram's chest (to see if he is alive). The color in Ram's chest keeps coming back in the spots where Veronica presses down. The character may be dead but someone failed to tell the actor's circulatory system.
Many of the extras in the school hallway scenes are clearly much older than high school age.
In the DVD extras Winona Ryder's bio states that her film Little Women was directed by Gillian Anderson. It was actually directed by Gillian Armstrong.
Heather Chandler drinks poison from a coffee cup that is clearly empty. This was so that actress Kim Walker would neither die nor be hospitalized at the time of filming so she wouldn't actually drink anything poisonous or harmful.
Despite the movie's setting taking place in the fictional town of Sherwood, Ohio, the movie's scenes were filmed in California, which is way out of place for the setting taking place in the easternmost state in the Midwest.