The problem was that the First Onebox was awarded for any posted link, no matter if it was or was not a Onebox. This happens because PrettyText adds class="onebox" to all links and then Oneboxer tries to parse all links with class onebox.
The change I made checks if the final document has any Oneboxes (Oneboxes are aside with class onebox). I will have another look and see what and why is it broken and come back with a change that fixes both of the issues.