Perhaps my head is not the way it should be then. It slides off with or without a blade. I have not yet tried a broken de blade. As usual I had an ASCO pro saloon half blade.Without using a tool, I think there's really only one way to break a DE blade in halves meant to be used for shaving and that way leaves the remainder of the tabs somewhat bent.
Having said that, I haven't had trouble with the head sliding off even after removing tabs completely, so I don't think it's a requirement. Then again, the blade does feel a bit stiffer with the bent tabs in there, so it's likely it was intended to be used that way.