If a top-level comment is hidden, an anonymous user doesn't see any of the replies to it. This is really confusing when there's only one top-level comment: the entire topic appears empty to anonymous users with no indication why, despite "4 comments" (see URL in bug).
To reproduce, start a new topic with one top-level comment, make tons of replies to it, then hide it.
This is similar to bug 63657.
One part of the fix: If some of the posts under a topic aren't visible to the current user, change the flow-topic-comments to "4* comments" with an asterisk and give it a title tooltip saying "4 comments, some not shown". I'll file a separate bug.