I missed the word new, thanks. Deleted the comment.
Edit: Apparantly I can no longer delete the comment. Stupid HN.
You cannot delete a comment once someone replies to it.
Nor should you. It breaks the conversation for anyone reading it afterwards and makes the replies essentially off topic.
As 'dpark noted, you can't delete a comment once it's been replied to: I believe this is to ensure that there aren't dangling comments. If the comment is still new enough (less than 2 hours old, I think), you may still edit the comment. For example, in this case you could chose to include your note about missing the word new in that comment itself.