The one that he apparently broke. If your really interested you can always trawl the internet. I'm off to get a plane to snow ravaged Austria.
However, it wasn't a rule. It was a precedent. And by definition, a set precedent is something that has been done that is different to what happened previously, which means that the precedent he broke was, at one time, just as controversial a change. It should happen more than it does, but politicians are cowards in this respect.