I just spent some time (a minute or so) to edit a new users first question. I found the question through the First Posts review queue. After editing the question someone else had already reviewed the same post and the "I'm Done" button was gone so I didn't get any review credit for my work.
This behavior of the review system rewards faster reviews and discourages real work. I suggest that the system is changed so that posts are locked to one reviewer at a time, or so that all the people who saw the review task is able to get it counted towards their badges.