Difference between "forget about it" and "forget it"

For me, forget about it is used more or less neutrally to mean Nevermind, don't worry about it.For example, you are at a restaurant and you order a Coke with your meal. Your waitress brings your meal, along with the bill, but not the Coke. She realizes it immediately and says she'll go get it, but you say, "No no, forget about it! It's not important", because you don't want to trouble her with writing up a new bill just for a Coke.

Forget it, on the other hand, is used negatively, with a strong hint of exasperation/annoyance/anger. For example, you are at a restaurant and you get into an argument with the waitress because she charged you for a Coke that you never ordered or even received. In the end, you say, "Ok, forget it. I'll pay for the damn Coke."