My favorite undocumented subreddit feature is broken in the alpha.

Going to /r/[subname]/comments in default reddit gives you a feed of all comments made in that subreddit as they are made. This is a super-handy feature for moderation as it doesn't require users to report new troll comments or for you to click through and update every thread to find them.

On the alpha attempting to open this page generates an internal server error (which seems to happen instead of a 404 for any invalid URL, for that matter).