Hello,all
I met a situation at a GPT. A player activated his Jace, Vryn's Prodigy's ability try to transform. However he made a mistake, instead of looter, he looks the top three cards of his library that like Anticipate he used last turn.
The player is first time to join GPT and not familiar with cards, I don't think he is trying to cheat. While he looked at the three cards, I stopped him. So he didn't put any of those cards into his hands and neither bottom of library.
Immediately I gave him a warning because of Looking at Extra Cards and made him put those cards back then shuffle library. But after thinking twice, I thought I was wrong, I should give him a GRV because the root cause is misunderstanding the rule of Jace. I am not sure which one is right and what is the proper remedy. Could you give me some advise?

I think you offered the best ruling possible.

Misunderstanding a rule should not make this a GRV. If that were the case (other than infractions involving intent or clumsiness) everything would be a GRV, and that would seriously limits the tools we could use to fix problems.

GRVs should only be used when no other infractions fit, and Looking at Extra Cards fits this perfectly.

I think this is HCE, unless if you and the players can confirm the order of the three cards he pulled, then it could be LEC as you ruled.

This shouldn't be GRV because it's not caused by other infractions fixable by public information. The philosophy behind this I think is that if the infraction could be pointed out and stopped by the opponent, before hidden information is leaked, like paying the wrong mana, or confirmed with him in the first place, then it is GRV. Otherwise the player draw those card and the opponent could not have possibly stopped him before he sees those forbidden cards, as what happened in your situation, then it's LEC or HCE.

Excuse me for my english.:)

IMHO if the cards touched his hand, its HCE. If he kept them in a separate set and the opponent confirms this, its LEC.