If you realize you called 911 by mistake just stay on the line and explain. Operators are understanding of mistakes, they will do some checks to make sure it really was a mistake as opposed to a real thing but you know the attacker is listening and then tell you bye. Don't make mistakes often, if it happens often they will look up the laws and throw fines at your for wasting their time, but for a rare thing it isn't a big deal.

If things are borderline and so you are not sure if this is 911 or maybe you can lookup some non-emergency number: just call 911 and explain. They will figure it out - better to call 911 when you didn't need to than call the non-emergency number and not get the help you need.

If you don't live in the US you may have some other emergency number. Know what it is. Generally the same rules as above apply.

A class 1 misd here. Also: dangerous, stupid, and anti-social.

But it happens all day long; sometimes people actually get killed; and misuse of public safety emergency responders is a serious vulnerability that very few seem to take seriously.