One time an attractive girl at a pool hall came up behind me while I was at the bar waiting for a drink. I was looking up at the big screen TV's smiling pretending to not be frustrated about standing at the bar for 10 minutes. She thought I was really into what I was watching and tried to strike up a conversation with me.

"Hi, What are you watching?"

At first I wasn't 100% sure she was talking to me, so I turned to see, sure enough, a cute girl looking semi nervous was trying to strike up conversation.

Since I was just blindly staring at screens waiting for a drink I suddenly forgot what I was doing, and started to sink faster than the Titanic in fast forward. The only way to save myself was to seem uninterested in her.. and she looked kinda sad about being turned down as she went back over to her friends.

My friends laughed at me. I felt like an idiot for the rest of the night.

TL;DR: Approaching a guy is great, but don't catch them completely off guard.