We had a pull request with two commits, one of which turned out to be unnecessary. After stripping the second commit in the source repository, the pull request broke.
It broke in the sense that trying to update it gave us a page where the reviewers field was unstyled so one could see that it's a select field. The preview below (showing the commits in the pull request) never finished loading.
Going back to the overview page, we could decline the request. When we did so, the overview diff showed the expected diff once again — it had been showing the "Nothing to merge, all changesets already exist in the destination repo" message after we stripped the second changeset from the source repo.
Let me know if the above makes sense, if not I'll try to reproduce it and post some screenshots.