I always wondered why all of my invalid flags were disputed, and this one is really strange:
Today I saw the first time a reason, well it is about this answer and it was flaged as "not an answer". At least I thought that this flag is wrong at is not "not an answer", that is why I flaged it as an invalid flag.
My flag was disputed with the reason:
disputed - flags should not be used to indicate technical inaccuracies, or an altogether wrong answer
But that was pretty much the same I thought, and I think that the "Invalid flag" is the right one.