A while back I was judging at my first Competitive REL tournament. I was a Rules Advisor at the time. I witnessed only one error, and didn't do anything except apply a fix because I was so used to Regular REL. But it has never gone from my memory because of what I learned. Whenever I read the IPG I think of that singular experience.
The active player attacked with
Sun Titan and pulled
Liliana of the Veil out of the graveyard to the battlefield. While still in his declare attackers step, he activated Lilian's second loyalty ability to try and get rid of his opponent's lone blocker, and that's when I stepped in. I asked him to put the counters back on Liliana and finish the combat phase before activating loyalty abilities and asked them to play on.
This seems to have been a GPE-GRV and the active player should have received a Warning since his actions went against the Comp Rules, right? Since I had just witnessed it and no further actions had been taken (the opponent hadn't sacrificed his creature, nor had he declared blockers), backing up was very easy, but I didn't get permission from the Head Judge to do it. His opponent seemed to be new to competitive Magic, which I realize doesn't matter when it comes to applying penalties, but should he have been given a Warning for the same thing, since it was public?