Well last semester I dated two guys at the same time, and lied to both of them about it.
But the worst I think would be when I was in high school and this guy came to my house to pick me up for a date. We went to school together and I liked him well enough I guess. I got out to the car though and for some freak reason I changed my mind, told him I needed to grab something I forgot in my house, went inside and never came back out!
He kept calling my house but luckily I was the only one home. I still to this day can't believe I did that...