I’ve unintentionally ignored another commenter. How do I undo this?

Go to washingtonpost.com/mycomments (make sure you’re signed in first) and scroll to the bottom where it says My Ignored Users. You should see a list there of any commenters on your ignore list and a “stop ignoring” link next to their names. Press that to remove them from your list and you’ll be able to see their comments again.