The Magic Infraction Procedure Guide is used only at tournaments with Rules Enforcement Levels (REL) of Competitive or Professional. These are Grand Prix Trials, Grands Prix, Pro Tour Qualifiers, Pro Tours, World Magic Cup Qualifiers, and the World Magic Cup and World Championship. Everything else, including Prerelease and release events, Friday Night Magic, and that random Thursday draft at your local game store, is held under Regular Rules Enforcement Level.

At Regular REL, there are really only two "rules." You aren't allowed to miss your own triggered abilities, and your opponent isn't required to remind you about them. The second one is a change that was put in at higher RELs a few months ago. Please note that this part applies only to triggered abilities. You can't ignore rules violations. Now, your responsibilities as the opponent are the same across different types of tournaments. If a triggered ability is forgotten (or any other game error is made) and the error is discovered later, players should alert the judge. The judge will try and fix the error if possible and, if not, the game will just continue. Regular REL is all about fun and providing a casual environment where players of all skill levels and experience levels can have a good time, so missing a triggered ability isn't punished too harshly.

At Competitive and Professional RELs, we still want you to have fun, but we understand that competitive behavior is the norm. Players come to compete and test their skills and decks against the best. As I've said before, Magic is at its best as a strategic challenge—a mental battle won by tactics and skill, not semantic trickery. We want to provide a clear roadmap to what is expected of players in various situations.

At Regular REL, there are really only two "rules." You aren't allowed to miss your own triggered abilities, and your opponent isn't required to remind you about them. The second one is a change that was put in at higher RELs a few months ago.

If a triggered ability is forgotten (or any other game error is made) and the error is discovered later, players should alert the judge.

As I've said before, Magic is at its best as a strategic challenge—a mental battle won by tactics and skill, not semantic trickery.

pourquoi sur chaque topic ça dérape à un moment ou un autre?

With the exception of a couple of fiddly bits that we'll talk about below, what the judge will do when you miss your triggers stays the same. Your opponent gets to decide whether he or she wants the trigger to happen or not.

4.4 Triggered Abilities
Players are expected to remember their own triggered abilities; intentionally ignoring one is Cheating. Players are
not required to point out the existence of triggered abilities that they do not control, though they may do so within
a turn if they wish.
Triggered abilities are considered to be forgotten by their controller once they have taken an action past the point
where the triggered ability would have an observable impact on the game. Triggered abilities that are forgotten are
not considered to have gone onto the stack.

une faille du système, qui lui exploite une faille de la mémoire. Mais c'est laid comme victoire, qu'est-ce que c'est laid...

A question for the judges out there. How much can I abuse missed triggers from my opponent? We're at competitive REL. The situation is the following:

It's the beginning of my opponent's turn who has a Dark Confidant in play and a Wurmcoil Engine. He is at 6 life and missed the Dark Confidant trigger (let's say he genuinely forgot). He draws a card normally and I immediately see that he missed his trigger.

I have Unexpectedly Absent in hand. Can I
1) play Unexpectedly Absent for X = 0 on his Wurmcoil Engine during his turn and
2) ask my opponent to put his Dark Confidant trigger on the stack afterwards? (which if it resolves without any other action will make my opponent lose)

At Competitive REL, the remedy for a missed trigger caught later in the turn is to give the opponent of the player who missed the trigger the option of whether the trigger is put onto the stack. So yes, if your opponent misses a trigger, it is legal to call a judge at a specific point in time where having the trigger get put onto the stack would be strategically advantageous to you.