If a post has too many comments to show, first you click "show (n) more comments," then you can post your own. However, after submitting the comment, the "show (n) more comments" link reappears. Clicking it just brings up the text box.
Note that this also happens when you visit a link to a comment that should be hidden (for example, when clicking on a reply in your notifications).
This is possibly a mobile-specific bug: repro-ed on iPod Touch iOS 5.1.1 mobile Safari.
Possibly a result of this bugfix: Add Comment disappears after editing a comment, or this one: "add comment" link disappears after voting to close a question as off topic with custom comment