I know a great deal of us have been in this situation. We date someone and it ends, either badly or on good terms. Either way, we forget about them for a little bit and move on to someone else. Later, we find out that said person is interested in someone new. "Who could this person be?" we wonder. Come to find out, it's your very best friend. I am very protective about my best friend, and most of the guys I have ever dated, I don't want them anywhere near her. But, if you are going to try and date my best friend, handle it the right way.
Guys, give it some time after we've dated before starting to even think about talking to my best friend. After a surmountable amount of time has passed, come and approach me. Say something like, "Hey, I know we dated awhile ago, and it ended good/bad, but I am very interested in (insert name here). I really like her; she seems cool. Would it be alright if I asked her out sometime?" Now, this may work. I may be alright with it. Sometimes though, I might say no. What if I dated you, and you cheated on me? Or you were terrible and mean or clingy and annoying? Why would I want to bestow that onto my best friend? But for the most part, I will be nice because you asked politely, and I will most likely give the go ahead to approach her.
That being said, last night was a clear violation of the above rule. A guy I used to have a thing with turned out to be a liar and a cheater and some kind of awful. Shortly afterwards, said person had the audacity last night to stop and stare at my friend creepily and smile at her, in a sad attempt to get in her pants. This all happened right in front of me, I might add.
If you're going to move on after someone, it's best not to date the best friend. If you plan on doing that anyway, don't disgustingly try to talk to them and dance up against them in a dark crowded bar while the friend you screwed over stands by and watches the sad sight. Although, I will say, we both got a good, long laugh out of it.