I have reputation enough to review First Posts by new users in two SE sites, and I found out a bug that is not particular to any of them, I believe this bug is in all SE sites.
How I came through it:
- In the "fist posts" review screen I voted up a comment to that post;
- The button "I'm Done" became enabled;
- I voted up another comment;
- I regret, and I removed the vote from the second comment;
- The "I'm Done" button disabled, but it shouldn't, because there is one comment voted by me.
At this point, if the page is refreshed the "I'm Done" button enables again, but IMO it shouldn't have disabled at first.
I couldn't test all the possibilities because it's not good to be playing in the review queue as we should take it seriously, but I believe that all "undo" action you take may set the "I'm done" button disabled, even if you actually did took an action that wasn't undone.
The bug might also be reproducible in other queues, but I didn't come into a situation that I could test that.