During the scene when Randy gets pulled into the van and killed, watch the reflection of the knife in the van miror when "Ghostface" repeatedly stabs Randy. Each time he raises the knife to stab again (you see the knife at least 3 times, i think 4) the blood pattern on the blade never changes. If that knife was being driven into someone's gut with that much force, the blade would probably be completely covered in blood, not streaked. Even if some of the blade looked clean, the drip patterns would be different with each stab.