Your friend's exs

There's a general guideline when it comes to most friendships:
Never date your friend's ex.

There's even another rule I was told, that seems compliment the first one:
Never date any of your ex's friends.

I've been wondering about these 'rules' for a while now, and I've been told again and again, especially by girls, that dating your friend's ex, or dating your ex's immediate friends is an absolute no-no.
Some people are absolutely fine with it, while some people avoid such associations like the plague.

It's been nearly 3 years since my friend broke up with his ex, and I've been friends with her since then, but he doesn't know about it. Whenever we go out for lunch or something, I feel as if something is developing between us that would take us further than just being friends. I wouldn't mind going in that direction, to be honest. But I still keep in mind the possible negative reaction my friend would take.

Has anyone else been in a situation where you found themselves going for your friend's ex, or them going for you? If you did get together, did it work out?

Jam it back in, in the dark.

that counts too, i guess.

this isn't just whether you think if you think it's ok or not, but whether it has happened to you, and what your response to it was.

There's nowhere I can't reach.

There's one reason I think why people would adhere to that 'rule', and that's to avoid those kinds of awkward situations.
Asking for approval from your friend; maybe having to go ahead with the relationship behind their back; sometimes they may even think you betrayed them!

This thing is sticky, and I don't like it. I don't appreciate it.