Factual error: At the end when the oil in the cavern is burning there is no sign of smoke. With a fire as large as it appears to be, everyone in there should be choking to death, yet they don't even seem the least bit bothered. Forest Whitaker should have also been roasting over the flames, yet he is unscathed.
Factual error: When Sil grabs Dan's ankle whilst he is hanging over the oil pit, she climbs completely out of the oil, she should be entirely aflame at this point as she is coated in oil and had to pull herself through the burning surface. Dan's leg would also catch fire from this and should be severely burned, not to mention the fact that he was just hanging over a fire.
Answer: You could probably make a compelling argument that, as an alien-hybrid, Sil is stronger than a normal person and thus was just able to forcibly cut through the finger with the pruners by "crushing" it. Also, this is where good old suspension of disbelief comes in. The pruners might realistically have broken at some point, but it's a movie, so we can "forgive" this bit of unrealism because it creates a compelling scene.
TedStixon