I get every day pleasantries horribly wrong constantly.
For instance, I'll drop off my money and tell the lunch people at my school what I want them to order, and when we're done they'll say "thank you!" I'll then spend a few seconds trying to think of what to say. "You're welcome" doesn't sound right at all for the situation, and "thank you" would just be repeating what they said.
If someone asks me "what's up" I usually say "Hi", which is horribly wrong, because I'm thinking "what's up" is a greeting.
If I happen to compliment someone, they'll say thanks. "You're welcome" isn't the thing to say for a thank you for a compliment, so I have no idea what to say. Normally I just do a small "hehe" chuckle, but it turns into a horribly awkward situation.
Then there's eye contact. Ugh, if I look at someone's eyes, I get all nervous and my heart rate jumps through the roof, and I can feel my face getting red. It's nothing conscious about making eye contact that makes me nervous. I have no control over it.
Because of these, I'm the socially awkward kid at the school, and usually very low in the social hierarchy because people think I'm being submissive. Something about my mind doesn't get social interaction very well. :(
TL;DR: I'm socially awkward. Halp.