Nothing. The author stated though that the merge had to be done:

> without losing the commit history and messages

Otherwise a "partial checkin" (which was exactly what "co-worker X" did) would have worked fine.

Ah okay, my bad.. I was mis-reading the article. I get it now. The topic branch commit history where the co-worker built up the branch would be lost if you just tacked on the leaf node to the master branch..