(at around 37 mins) The girl with the yellow swimsuit stabs the beaver in the head. Several seconds later, when the band is examining this beaver, the knife has moved - now, it's not in the beaver's head, but in his back.
When the three girls are walking around looking for beavers in the first half of the movie, Zoe is walking around topless, Mary asks Zoe if she can cover up while they're walking. Zoe says that her top is on the opposite side of the island and mockingly pushes Mary and asks if she can lend her her top. While she is pushing her, you can briefly see (in a split second) that Zoe is wearing a yellow top. Next seen, she is topless again.
When Sam and Mary are in the bathroom, he takes off all his clothes. When the beaver attacks him in there, he is suddenly wearing his underpants again, with no reason to have put them back on.
When the girls first arrive at the cabin, just before they go inside, Zoe puts down her little dog, Gosling, and tells him to go play. He runs off to the woods. In the next shot, as they walk in the door, Zoe is carrying Gosling under her arm.
When Jen hands the phone to Sam, the way she holds it changes between shots.
When Zoe is driving off with Mary in the end of the movie there is no wind blowing on Zoe's face/hair despite the fact that half the windshield is gone.
The barrel of toxic waste at the start of the movie was obviously an empty barrel - full barrels wouldn't float that high in the water (if at all).
During the Zombeavers' first attack they are seen breaking up through the boards of the raft. Despite being in the water the puppets are clearly bone dry.