Sometimes old answers were correct at the time they were written, but the march of technology has made them obsolete.

For example, the command-line video encoding tool ffmpeg used to have a 'vhooks' function, but that has been replaced by an entirely new syntax, making this answer incorrect for all recent versions of the program. However, when I flagged the answer as such, my flag was rejected on the grounds that

flags should not be used to indicate technical inaccuracies, or an altogether wrong answer

So my question is: what is the correct response to this kind of obsolete answers? Downvote & leave a comment? Edit the question to correct it (would seem to be little too drastic a change for an edit)? Something else?