gistfile1.txt

I created a rewrite endpoint called 'filter' which I'm going to use to toggle between 
the standard blog post view (most recent posts), and most comments view (/filter/comments)

The code is really simple: https://gist.github.com/1285290

And it works great for the main blog page:

http://localhost:8888/dev-bachis/blog/ displays all posts in order of post date
http://localhost:8888/dev-bachis/blog/filter/comments displays all posts in 
order of most comments

But it 404s on category pages

http://localhost:8888/dev-bachis/category/uncategorized/filter/comments - 404
http://localhost:8888/dev-bachis/category/uncategorized/?filter=comments - works


So right now I'm not using the rewrite endpoints and just adding query vars to the 
end, but it would be great if the endpoints worked for cleaner url's. 

Any ideas? I thought maybe the name 'filter' was causing an issue so I changed 
it to 'blog-filter', flushed my permalinks and still had the issue.

Don't you need to add new rewrite rule via "rewrite_rules_array" filter? Like this:
add_filter("rewrite_rules_array", "my_rewrite_rules");
function my_rewrite_rules($incoming){
$rules = array( 'my_regexp_url' => '?raw_url' );
return $rules + $incoming;
}

I haven't tried that yet, but I thought add_rewrite_endpoint would handle creating the rewrite rules. It works on standard posts, pages, and the main blog without adding any rewrite rules.

You're probably correct there, I never used endpoint before and looking at the codex it seems should be handing rewriting as well.