The story goes like this:
The guys, who are both under SM Entertainment, used to spend a lot of time together before their debut. They lived and practiced together.
There was this one time when U-know and Heechul went to the cafe together. U-know’s girlfriend spotted the pair and saw U-know’s companion from the back. Since Heechul had long hair at that time and looked feminine, she mistook him for another girl!
U-know had to do damage control after the incident, explaining to his then girlfriend that Heechul is a guy, who is just a friend. But they eventually broke up.
Oh man, poor Heechul. I wonder if he’s taking that “look like a girl from the back” thing as a compliment or insult? :-P
Source & Pic credit: Sports Seoul with translations by Joe Gimm