Have you ever had the feeling that you put the wrong thing in the wrong envelope? It happened to me today when I sent thank you cards to my interviewers. I'm sure I put them in the right ones, but I was just uneasy about putting them into the mail.
It would suck to look like a total idiot that doesn't pay attention to detail, especially over a thank you note.
Oh well, I guess that's just me being neurotic.