1:A phrase commonly said after one person in an argument has said something thinking they know what they're saying or thinks that they are right and another person corrects them making the person look stupid.
2:A phrase said by one person when a person has just made oneself look stupid.
Tim:"Hey did you know Frank is wearing a red shirt?"
Dan:"No! I am positive he is wearing a green shirt! I just saw him!"
(Frank walks around the corner and everyone sees he is wearing a red shirt)