If you changed your permalink structure, your site may experience some crawl errors in Google Webmaster Tools.
Unfortunately, these errors cannot be fixed by the autosearch 404 page.
First, try to find the root of the error by clicking on the link in Webmaster.
In this example, older post still have absolute links pointing to the blog’s pages with YEAR/MM in their links, even though the new permalink structure doesn’t have dates in the URLs.
So, to fix this errors you have to find and replace all old URLs in posts to new ones. This may be done by an SQL query, editing of the blog posts export file and importing in back or manually in the WordPress console.
For the latter in this example, simply search for <a href="http://domain.you/2002/02/ in the posts console page, and manually edit out dates in URLs of the other posts links inside each post in the search results. Be sure not to edit out the URLs for the media files.
As for the comments/feed not found error, this one is much simpler. WordPress has a built-in feed for comments, the address of which is advertised in the header section of your blog template. RSS-enabled browsers, like Firefox, may use this information for subscriptions. The problem is, this comments feed link is always in the header, unless you block it in the theme functions. But the feed page itself is available only if you have comments in your blog. If you don’t have a single comment in your blog, the feed is nor generated. No feed means no page, that’s why you’re getting a 404 error for domain.you/comments/feed.
To fix it, simply post a comment on one of your blog posts yourself.