When you're not sure wether to put hahaha or haha. You hope the person will take it either way. Many would think there is not a big difference between hahaha and haha but they are wrong. Haha is more of an unnecessary pause while hahaha signifies a joke was said that was actually worth a chuckle. Hahah is a good compromise for the indecisive.
Guy 1: Hey, so I totally hooked up with your mom last night.
Guy 2: Hahah, no way.