I remember watching the post-fight celebration when Brock Lesnar defeated Shane Carwin. He looked relieved. That was the first thing I noticed about his facial expressions. He didn't look like he was on a mission, or confident, or anything. He honestly looked like a kid who just had a shot from the doctor and realized it wasn't as bad as he thought it was going to be. And his next fight, he lost to Cain Velasquez. I don't know if the two are related at all, but it was certainly a weird facial expression to have.
Also, a couple years ago before Kimbo Slice was defeated in a Strikeforce X fight (I may be wrong about the brand) I noticed his facial expressions were really weird pre-fight. He looked in the camera and and was frowning as though he was angry, but it looked like a fake frown. Like he actually wasn't upset or angry at all. It was very strange. He lost his fight too and is now out of UFC entirely.