This is a bit of a corner case, but it is a bug nontheless:
Steps to reproduce:
1) Create a repo (repoA) and make a commit to it (commit1). 2) Fork repoA to repoB and make an additional commit to repoB (commit2). 3) Browse the changelog history of repoB and set the review status of commit2 to "Approved". Do NOT push repoB to repoA. 4) Fork repoB to repoC and add a commit to repoC (commit3). 5) Change repoC's parent to repoA. 6) Try to open a pull request for repoC (requesting that repoC be pulled to repoA -- this pull request will contain commit2 and commit3).
Result: The pull request will fail to open -- you will get an error about commit2 already having a set review status. If you browse to the changelog for repoC and look at commit2, however, it will have a status of "Not Reviewed"
From an outsider's perspective, it seems something is getting transferred, but not completely, about the review status of changesets when you make a fork.