Albert is playing against Nora in PPTQ. Albert has lost game 1, and they are moving to game 2.

Albert swaps some cards in from his sideboard, presents, then draws his opening 7. There’s only one land, so he decides to mulligan. He puts his hand back on top, says “Hmm, actually…” and swaps one more pair of cards from his main deck and sideboard. He then shuffles thoroughly, presents to Nora, and draws 6. At this point, Nora says, “Hold on a minute. I don’t think you’re allowed to sideboard between mulligans.”

Albert responds, “Uh oh. I think you’re right. Judge!”

What do you do?

Judges, feel free to discuss this scenario on Judge Apps!

[expand title=Answer]To determine the appropriate infraction, we must first determine which rule
or policy has been violated by Albert’s actions. In this case, Albert made
a change to his sideboarding choices during the mulligan process. While
some aspects of the pregame procedure are contained within the
Comprehensive Rules, when a player is allowed to sideboard during a
multi-game match is not one of them. So we know that this isn’t a Game Play
Error of any sort.

If this is a violation of the Magic Tournament Rules, we are in the
Tournament Error category. Some have explored the possibility that this
error is covered as a Deck/Deck List Problem. However, we neither have an
illegal deck presented at any point, nor a sideboard change occurring
during a game – we are still in pregame procedures! What this leaves us
with is an uncategorized Tournament Error, for which there is no infraction.

Explain to the players that they may not, in fact, change sideboard
decisions after a mulligan. Ask them to please play more carefully and call
a judge before they take an action they are unsure about, rather than after. Then instruct them
to play on.
[/expand]