When someone is so defensive about something being true/false, that it is obvious that they want the opposite.
Dan: LOL you have a crush on Kyle, don't you Erica?
Erica: NO! What are you talking about! That's the stupidest thing I have ever heard! Kyle is the last person i would ever like, EVER! He is definitely NOT the cutest boy I have seen in my life, and I definitely do NOT want him in my pants!
Dan: Tsk Tsk *waves finger*. does someone have a guilty conscience?