Rav said:
The problem with that is that you can not cleave someone who is behind your first target... you just don't have the reach, as you just threaten the squares 10 ft away form you. Sometimes you might also want to cleave someone in a 90 degree angle 15 feet away... It isn't broken or anything, it's just silly. I don't see why they didn't make cleave B and S only. They have those rules with "keen" as well for instance... I know, technically they are flavour rules, but they are flavour rules that make a helluva lot of sense.
Rav
I can rationalize it enough in that the target ducked or tried to dodge to the side, lining up with the other. I have about the same trouble imaging a cleave by a club - without redirecting the swing it should not hit another.
In that cases I take the results from the crunch rules, and apply flavor to it. If the barbarian cleaves then the two monsters attacking were behind each other for that split second of his thrust. No problem for me.
Trying to curb options for "realistic flavor" reasons while adhering to an abstract 5-foot-grid combat system is somehow not to my taste.