Not binding properly. Not maintaining the bind. Boring. Pulling down the scrum. Skew put-in by the halfback (hardly ever called any more). Also mouthing off to the referee and various forms of foul play, but those are not scrummaging problems, per se. There are some oddball ones,like repeated failure to take the proper mark, but no one would try to pull that on a Test-level referee more than once.
Most of the penalties given by the ref are guesswork. Something went wrong so I'd better give a penalty. At the next scrum the penalty will go the other way. It only goes as clearly as it did in that match when one side is really on top of the other. Even then the commentators will ask "why would the prop pull down the scrum when they're going forward?".
