When Lizzie's mom first finds out that Mark hit her, she and Lizzie argue in her bedroom. The blood on Lizzie's face appears and disappears as the scene goes on.
Elizabeth's hair changes length from scene to scene. It also changes from dark brown to light brown & back again as the film progresses.
Elizabeth's weight fluctuates from scene to scene. This is especially noticeable when Mark & Elizabeth are play-fighting during Kelly's movie night. She is somewhat heavy. But when she & Mark are making love, which is only days later at most, she is noticeably slimmer.
When Elizabeth walks down the school hall after making love with Mark, several students look directly into the camera.