The first example for the infraction Looking at Extra Cards (L@EC) is "A player accidentally reveals (drops, flips over) a card while shuffling their opponent’s library." This is straight forward enough, and it is probably one of the two most common ways that this infraction occurs. The other being the second example: "A player accidentally reveals (drops, flips over) a card while shuffling their opponent’s library." It's almost like these are good examples or something!
As a Scorekeeper, I see judges write descriptions for infractions on the backs of slips, and the first example for L@EC is one I've seen variations of plenty of times. But "flipped over card while shuffling opponent's deck" as a description tells less than half the story. Think about it this way. Seeing a card in your opponent's deck confers an information advantage, but the potential for that advantage can vary wildly depending on when this infraction takes place.
The most impactful timing is when a player shuffles their opponent's deck prior to the first game of the match. Barring information from scouting, which could be intentional or just from sitting next an opponent in the previous rounds, players don't have knowledge of what deck archetype their opponent is playing at this point in time. Early game decisions, particularly mulligan decisions, are made in this haze of ignorance.
Flipping over a card while shuffling prior to Game 1 can lift that haze. Imagine this happening in a Modern match and a Baral, Chief of Compliance gets flipped over. Aha! U/R Gifts Storm! This information can certainly inform a player about the kinds of hands they should keep. Even something as innocent as a land can provide information. For example, Horizon Canopy is featured primarily in creature-based strategies.
This type of information gain loses effectiveness the deeper you get into a match because a player will naturally see more cards from their opponent's deck through game play, and the deck archetype will reveal itself somewhere in the middle of the first game.
Any time we speak of the potential for advantage, we should also consider the potential for cheating. Generally, more advantage means more potential for cheating. A player committing a L@EC infraction for flipping a card while shuffling their opponent's deck during Game 3 has almost zero advantage, and we can quickly dismiss cheating based on context. But when this happens prior to Game 1, it deserves more scrutiny. So what does that scrutiny look like?
First off, ask about the timing in the match. Sometimes this is obvious. If you get called over at the beginning of the round and there are no permanents out, you know that it's prior to Game 1. But specific timing still matters here. Is this during a mulligan shuffle? Did the opponent keep? If the answers are "yes" and "yes," you can tick the advantage bar down a notch because the player can no longer use this information to help with their mulligan decisions. These are small pieces of information that can help you during a quick investigation.
The thing with this type of investigation is, it's highly unlikely to yield a guilty verdict. For one, most of these infractions are innocent mistakes. Second, a player intentionally committing L@EC to see opponent's cards most likely looks like all the innocent players. Unless you happened to be watching and saw a very suspicious shuffling style, it's hard to sort the malicious from the innocent via a post-incident interview.
Thus, our best line of defense becomes recording the infraction and looking for a pattern. And this is where I return to my original point; judges need to write down when the "card flip L@EC" takes place. Was it during a mid-game fetch? Was it shuffling up before Game 2? These both rate on the innocent side. But if we are recording infractions to look for patterns of potentially malicious behavior, this is an absolutely important one to record in more specific detail. If a player commits multiple L@EC infractions, all while shuffling prior to Game 1, that's a lot of potential advantage, and someone that I'm a lot more interested in talking to or watching while they shuffle before their next Game 1.