A Roulette Wheel and a Piano Playing Dog - S7-E2
Character mistake: Sheldon likes his pretzel and says "Ich bin ein Heidelberger." He makes the same mistake President Kennedy made when he said "Ich bin ein Berliner", which means "I am a jelly donut." The correct German is "Ich bin Berliner" or "Ich bin Heidelberger."
Ants on a Log and a Cheating Winker - S7-E4
Other mistake: Sheldon (in the voice-over) says "it really brought what's his name and I closer together." The grammatically correct expression is "brought what's his name and ME closer together", and a stickler like Sheldon would not make that mistake.
A New Home and a Traditional Texas Torture - S7-E12
Factual error: When Sheldon is visiting the comic store for the last time, you can see a comic called Future's End. That comic didn't come out until 2015, but this episode is set in 1994.
Answer: Some people just have a dislike for someone that can't be explained. Could be as simple as the way he's dressed.