Back in college, I was taking an ethics class, and one day we were going over Marx (yeah, I don't know why he came up in ethics class, either). The professor made a mistake in explaining one part of his philosophy, and I raised my hand to correct her. I don't remember exactly what it was about, since this was over ten years ago, but she told me I was wrong, and not to interrupt her class. Well, a couple days later, I showed her a quote from Marx himself setting the record straight (in rather harsh language - what can I say, it's Marx), because several people had the same misconception in his day. This made her even more irate. Another student later tried persuading me to apologize to her, but I wasn't having it. Nobody ever acknowledged that I was right, so in spite of that, check out this flex, Dr. Wrong!

Edit: I just remembered another one. In a former programming job of mine, I was arguing with my boss over one piece of code. I said that the code wouldn't run, and he said that it would. So, I ran the script, and it threw an error, as I expected. It wasn't that difficult to understand; I think a computer science student could have understood why it wouldn't work. But, I chose to act meek, and say, "Huh, looks like it threw an error." But now that I don't work there anymore, I can confidently say, "Error: Variable manager_intelligence not found."