The format is modern. A mulls to 6. Keeps 7 cards, forgetting to put one back. A is on the play, plays a land, and goes “oops, Judge!”

MPE states: “Once the mulligan process is complete and the game begins, any excess cards arising from an improper mulligan should be treated as Game Rule Violation — Hidden Card Error.”

Ok, great. However, HCE says:

“A player commits an error in the game that cannot be corrected by only publicly available information.”

With the London Mulligan, the players CAN correct the information with only publicly known information. Possibly.

A few wrinkles:

1) Leylines
2) Force of Negation

With a player playing a land, and realizing they forgot to put a card back, what remedy and fix do we apply? Assuming nobody has dropped any leylines, and no interaction has occurred.

Not Official - my understanding is that the MPE line of text you quoted is accurate, and should guide you here. The mulligan process is complete, and the game has started, so any extra cards should be fixed with HCE.

We don't want to let players apply the london mull fix themselves, after the game has started, because it could give them a huge advantage. Part of the fun and skill in Magic is deciding what cards to keep during mulligans, and weighing how likely a card is to be bad vs a given opponent vs how good it is vs others, and so on.
If we allowed players to apply the london mull after the game had started, they could have extra information about whether they wanted to put their Fatal Push or their Cryptic Command on the bottom, based on what land their opponent played on the first turn. We want them to make those choices before they get any info about what their opponent is playing.

Edited Brook Gardner-Durbin (July 20, 2019 02:25:28 PM)

The text says it's MPE, so I'm (unofficially) pretty sure the intent is for it to automatically be HCE.

That said, I strongly disagree with that fix and hope it changes.

Brook, I get what you're saying about potential for advantage by waiting to bottom a card, but you can also gain potential advantage by waiting to see what your opponent plays or what you draw next turn before discarding to hand size limit–but we still fix “failure to discard” with a GRV partial fix, rather than a []. That's because the governing philosophy of HCE is as follows:

Be careful not to apply this infraction in situations where a publicly-correctable error subsequently leads to an uncorrectable situation

Keeping too many cards, like a failure to discard to the hand size limits, is publicly correctable. The opponent, who has a responsibility to maintain the game state, had the opportunity to catch the error when it was still fixable. Yes, subsequent actions made the error much harder to fix–but according to HCE's philosophy, that isn't supposed to result in an escalating remedy. Unless there are complicating factors I overlooked (entirely possible!), the mandate in MPE to use HCE appears to be contradicted by HCE's own mandate not to use HCE.

Originally posted by Brook Gardner-Durbin:

Not Official - my understanding is that the MPE line of text you quoted is accurate, and should guide you here. The mulligan process is complete, and the game has started, so any extra cards should be fixed with HCE.

We don't want to let players apply the london mull fix themselves, after the game has started, because it could give them a huge advantage. Part of the fun and skill in Magic is deciding what cards to keep during mulligans, and weighing how likely a card is to be bad vs a given opponent vs how good it is vs others, and so on.
If we allowed players to apply the london mull after the game had started, they could have extra information about whether they wanted to put their Fatal Push or their Cryptic Command on the bottom, based on what land their opponent played on the first turn. We want them to make those choices before they get any info about what their opponent is playing.

To be clear, it's the player going first who noticed - his opponent has done nothing all game, so there's no information gained or real potential for abuse that I can see (Again, ignoring the existence of leylines, which I'm not sure can be done). If it was the player on the draw who noticed, then yes, it would be more problematic.