A syndrome that is caught when someone is around a Grammar Nazi who always corrects incorrect usage of "your" for "you're." After being around Grammar Nazis, people will oftentimes develop a case of "You're" Syndrome.
These people will use "you're" regardless of context, including when "your" is actually the appropriate word. This is an act of carelessness and/or misunderstanding.
Person 1: Hey, can I use you're computer?
Person 2: You want to use I'm computer?
Person 1: No, I want to use you're computer.
Person 2: Wrong form of "you're." "You're" is only used in place of "you are."
Person 1: What do you mean? I thought it was always "you're."
Person 2: Dude, you've got a severe case of "You're" Syndrome.