When Cindy confronts Ziggy about almost getting kicked out of camp, she says that they had the same conversation the previous week, and the week before that, but at the beginning of the story, the narrator said that the picture showed the sisters on the first day of camp and a week later her sister was dead, so they wouldn't have had weekly conversations about her behavior at camp.
Ziggy's bandage on her "burn" (opening 5 minutes of the movie) on her right arm, changes locations on her arm throughout the movie.
When Joan is in bed after having sex with Kurt, she is uncovered but in the next scene, the sheet covers her.
(At around 1h27m) Alice is demonstrating that she is able to walk and put weight on her broken leg. Her break was the most severe type, a compound fracture, where the bone is completely severed and has pierced the skin. It would be impossible to put any weight on it, nor walk, as there is no supporting structure, not to mention the severe pain.
When Ziggy is carrying the paint can to replicate the Carrie blood scene, it is obvious by the way she's carrying the paint can there is no paint in the can... it swings freely by her side.
The snake featured in the Science and Nature cabin appears to be a juvenile bullsnake (Pituophis catenifer sayi) which, according to the Ohio Department of Natural Resources, is not native to the state in which the movie takes place.
When Ziggy and Gary are in the camp bathroom, Sheila is passed out next to the toilet. But when Ziggy tells Gary that Cindy is in the toilet, Sheila is no where to be found.
(around 1h 31m) Cindy and Ziggy dig up a rock that's been buried a few feet under ground. The rock was impossibly clean of dirt, despite having been buried in moist earth for an extended period of time. Buried rocks do not come out of the ground clean.
The story being shown is from both sisters' point of view, with details that either one couldn't know about the experiences happening that night. So it doesn't make sense that C. Berman, as the only surviving Berman sister, would know all of these details.
At 1.31 Ziggy and Cindy collide heads under the tree when Cindy pulls Ziggy away.
In the beginning of the movie when older Ziggy is pouring a glass of Jim Beam whiskey the bottle says it's the 200th anniversary but the 200th anniversary of Jim Beam whiskey wasn't until 1995.
During the Camp Competition rally, on counselor says comments that another is "Shagadelic". The setting is 1978, but the first use of the term shagadelic is in Austin Powers International Man of Mystery (1997) - 19 years later.
At the end, Josh and Deena are surprised to find out that Berman is Ziggy and not Cindy. Ziggy was the one telling the story. She would have been using "I" pronouns when referring to herself.