Clearing out opponents
One of the more nefarious tactics used is the clearing of opponents who are not taking part in the ruck or post-tackle situation. It happens consistently in the southern hemisphere and is a tactic referees are very poor at picking up.
It appears that there is either a lack of will or lack of quality personnel to deal with these indiscretions. It causes frustration for the non-offending team, who are trying to defend legally, and often leads to retaliation as a result. Teams get away with it quite often, and hence some big holes are opened up for the attack to exploit.
The clean-out from the side, when the support player overruns the carrier, is not well refereed.
It is not a popular penalty to give against the attacking team, but I am also certain that a good defensive hit needs a reward as well, no matter the interruption to the flow of the game. I really think accuracy and good communication override the need for a spectacle.