In another online game I used to play every few months or so a faulty patch would come out, but we didnt have to re-download the whole client.
The fix was simple.
We opened up a config file and changed the version number to the number BEFORE the patch. this forced the auto-updater to re-download the patch.
You could argue that a manual download already being available is enough but then again this would be another option for people to follow should they choose to.