Pull request corruption/brokenness

Something really strange going on with this pull request:

https://bitbucket.org/spookylukey/semanticeditor/pull-request/6/use-only-the-first-part-of-the-language/diff

1. When I first looked at it, it contained a pull request for the following commit:

https://bitbucket.org/paparent/semanticeditor/commits/9aa8b59c393bf329288d21d4cac79f7d883c566f

The diff displayed as on that URL (changes to single file, semanticeditor/widgets.py)

1. I pushed some more changes to my repo (spookylukey/semanticeditor), in this case 112e232 and 050ceca
2. I refreshed the pull request page, in case it showed some problems, but it didn't.
3. I pressed 'Merge', and it came back with an error "Unexpected merge error" (or something like that, I closed the window I'm afraid).

However, from the history, the merge had succeeded:

https://bitbucket.org/spookylukey/semanticeditor/commits/5c74d9f2ac67fda8615f6e28c7245a0b5aed2470

1. Going back to the pull request, it now shows a different diff - I think a reverse diff of the other commits that I had added. This is totally confused.