When I first watched the original "The Thing" as a little kid (stayed up late without parents knowing, and it aired on Hollywood)
I had to sleep with the lights turned on for quite a few months..
This scene is still to me the creepiest and scariest thing I have ever seen in any movie I can remember. That and the mutant scream, that scream just penetrated your damn soul.
Warning: This scene is very explicit, watch at your own risk