Carbon Dating and a Stuffed Raccoon - S2-E7
Continuity mistake: Sheldon places his membership cards down on a flat surface. He puts Radio Shack first, followed by Star Command, almost entirely covering it, leaving just a small section of red along the top. When the shot changes to a wide-angle one, the Star Command card has moved down and more red is visible above it. (00:01:00)
Answer: Some people just have a dislike for someone that can't be explained. Could be as simple as the way he's dressed.