Bert pisses off the same rednecks he would later depend on to save his life, by stealing a candy bar from them at the beginning.
Bert and Jeff make a bet to see who can last the longest drinking nothing but beer. Bert forgetting about this bet and drinking a sip of water ultimately killed him.
The first person Jeff and Bert find when they go looking for a phone so they can report that they had to incinerate the sick hermit, that person turns out to be the hermit's cousin.
Paul actually killed the girl he wanted to score with by urging her to drink some (tainted) water when she was upset.
When Paul walks in on Jeff and Marcy having sex, he advises them to practise "safe sex." He should've followed his own advice much later when he had sex with Marcy, because she had the same disease by the time they hooked up later on.
He actually tells Marcy he's concerned about not wearing a condom when they start screwing. But then he just takes her word for it when she tells him it's safe. It didn't work out too good for him in the end...
By the time she has sex with Paul, Marcy's infection has progressed far enough for his squeezing of her back to leave red welts. How squicky would it have been if they'd had sex only a couple hours later - like around the time she was shaving off her leg flesh - and her back started peeling off in his hands while she was still screwing him?
In Cabin Fever 2: Not that it matters to poor Paul at this point (because he's already dead), but how bone-chilling would it be if Paul had found out the only way to survive the disease was to amputate the body part that first contracted that disease? Remember, he actually got sick from a bareback sexual fling in the first movie.
Same thing applies to Alex from Spring Fever. He contracted the disease from getting a blowjob. Although, considering he asks where his dick is, it seems his penis would've fallen off either way.
There is something truly wrong with Old Man Cadwell's jolly manner at the very end of the movie, considdering that his entire staff had been killed only the previous afternoon, and his storehand's son was in intensive care in critical condition. Even if he wasn't entirely aware of what had happened yet, you'd think he'd at least be worried about why all his staff are missing.
The shots of the sun rising over the lake that roughly punctuate the start of each act go from featuring a pristine lake to an increasingly debris-covered lake. The deterioration of the beauty of the lake represents the deterioration of both the physical beauty of the students and the deterioration of their friendships.
Marcy's hair is straight on the first day and more unkempt the following days. She takes frequent baths - at least three after the first night - and even if she didn't wash her hair, the evaporating water would still stop it being straight. And she's clearly not in the mood to spend time straightening it the next couple of days.
Jeff refuses to eat anything after Karen's infection is discovered, saying "what if Karen used that plate?" Marcy responds that she washed all of them. Since the infection is carried in the water, Jeff refusing to eat anything is why he's the only one that doesn't get infected. This would also explain why Bert gets the infection sooner than Marcy or Paul; since he ate something off the plates and drank some of the water.
It's not the sort of detail audiences tend to pick up the first time around, but on subsequent viewings, a lot of viewers get hung up on the mystery of why Paul suddenly has a wristwatch on halfway through his sex scene with Marcy, when he clearly wasn't wearing it when they started. Naturally, there are Fan Theories...