We've been getting a lot of obsolete flags on comments in recent months. Comment clean-up is good, but these tend to be hard to process because we don't know why it's obsolete. This is particularly hard when part of a long comment thread is flagged this way. As a moderator, I don't want to have to dig through the edit history trying to match up edits with flagged comments, or try to understand a long argument among a few people. So I'm asking for your help.
Sometimes it's pretty obvious -- a comment is @-addressed to someone whose comment is now gone, or there's an exchange like "please fix X" / "thanks, done". Feel free to keep flagging those as obsolete.
But if an uninvolved party, somebody who hasn't been watching the comments and revisions go by already, and perhaps someone who is not expert in an obscure point or advanced concept that's being discussed, sees an "obsolete" flag without more context, it's pretty hard to evaluate. Please help us out. Use an "other" flag on the comment to explain, or if there are a bunch of comments you can flag the post instead and describe the problem.
Flagging the post is especially helpful if you are one of the participants in an obsolete conversation. This solves the following problem: you don't want to delete your own comments until the others go, and you can't flag your own comments, so you flag the other person's comments and rely on a moderator to notice what's going on and delete yours too. A custom flag is a great alternative to that.
Except for spam/offensive, there's actually no particular benefit to using the built-in flag reasons for comments; all comment flags go to the same place, the moderators' flag queue. If you have any doubt about whether a built-in flag reason is clear enough, please use a custom one. This is different from flags on posts, where the built-in reasons can send a post to a review queue or automatically delete a post as spam or offensive. For posts, please do use the built-in reasons when you can; it enables the community and not just the moderators to act.
Thanks everybody for helping to keep the site clean. We are one of the more comment-heavy sites on the network (average comments per post), so it's good that people are keeping an eye on this. If you can give us a few more hints to help us understand your flags, that'd be great.
Related feature request on Meta.SE: Can we add more information to "obsolete" flags?.