I get that editing a comment when someone has replied to it could cause a lot of confusion, but if it's still the last on the question/answer we should be able to edit it. For instance, today I commented and realized when it was too late that I forgot to use the @username so the person would get informed. Or what if I posted a comment when irritated or even when not irritated, looked at it again 15 minutes later, and realized it sounded angry when I didn't mean it too. It's true I could delete and repost, but if it's only one sentence that got messed up that seems like overkill. Also, the person who asked the question would get 2 messages for practically the same comment. I think we should be able to do this.
Forget to @ someone?
Hit the edit button and add it back in.
Regret posting the comment?
I hate to be informal, but that's what the delete button is there for, brother.
I understand what you're asking for, but I don't understand why your arguments suggest needing what you're asking for.
We already have the ability to easily handle the cases you raise.
If you really need to edit your comment and it's too late, delete it and post an updated version.
Seriously, it takes considerably lesser effort than that which would go into implementing this feature.
It's a great idea, but I'd rather see the team working on other stuff, given that we can already work around the proposed issues without much deliberation.
Also, the person who asked the question would get 2 messages for practically the same comment.
The other answer already addressed how you can edit your comment to @someone/delete the existing comment. But I want to write a bit about why the particular bit of reasoning I quoted above is a bad idea for this feature request too.
While not getting a notification when someone only corrects a typo may be nice... allowing last comments to be edited without anyone getting any type of notification would be open to a lot of abuse as well. Unlike a lot of actions on posts, including editing, comments do not 'bump' posts. And the follow feature at this moment doesn't notify of edited comments either. So a last comment could be edited into something totally inappropriate, without even the OP of a post/the person the comment is @replying to getting a notification and a signal to flag the inappropriate comment. There's just too little chance for community moderation with this feature.
I'd rather have one notification too many than have a feature that allows inappropriate comments to fly under the radar like that.