Very close friends, I would be inclined to think at one point, one person or the other will develop feelings except when:
-Both are close friends due to some other circumstance (friends through boyfriends/girlfriends)
-They are a part of a larger group of friends.
-They are completely the other person's "non-type." For example, if I became close friends with a heavily bearded guy who had a strong non-attraction to Asian nerd girls.
Edit: -If one of them are gay. x3
This is what makes it extremely rare, but not impossible.
Acquaintances, work, professional, study, and online: I believe it is possible and common.