I have noticed once in a while that when I flag something, I get a warning that some of my flags have been declined recently. I wonder what the risk is of being overly flag-happy, but that is not what I want to ask here. I looked through my flag history, and noticed that there are three different statuses for a flag: helpful, declined, and disputed? What do declined and disputed mean? How are they different?
Declined flags were found (usually by a moderator) to be without merit. They result in the warning that you saw.
Disputed flags were dismissed (by an automatic process based on reviewers' actions, etc) without a decision on whether they had merit or not. They do not count against you.
For details and examples of situations leading to such outcomes, see