Revealing mistake: When Sil fakes her death, the breaker box explodes before the car hits it.
Revealing mistake: When the team is at the motel and seeing the tape of Sil on it, when the motel clerk tells them where she went, Michael Madsen can be seen smirking, obviously breaking character.
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