Blood Moon on the battlefield.

AP plays a shockland, tapped, and says “Go”.
NAP untaps.

What do you do?

A rule was broken: either AP played a card taped when it would be untapped, or AP passed priority with many in his or her mana pool without announcing it.
Intervene, investigate to know which one it was and if NAP was aware of that, aware of the appropriate rules, and broke it on purpose to gain an advantage.
Apply the appropriate fix if any (most likely rollback to the point the land was played, make it enter the battlefield untap, and AP have priority), and resume the game of NAP hasn't been disqualified. Educate between games, and make a note that an annoucement at the start of the event may have been appropriate given the format.

- Emilien

Originally posted by Emilien Wild:

A rule was broken: either AP played a card taped when it would be untapped,or AP passed priority with many in his or her mana pool without announcing it.

Emilien, would you step into the match and investigate if a player played a Swamp tapped and then passed the turn?

The answer is in your quotation.

- Emilien

Emilien, I asked because my reaction to this situation was to say this was a legal game state, and the player not announcing their floating mana was not a big problem, and covered by the IPG's sentence “If a minor violation is quickly handled by the players to their mutual satisfaction, a judge does not need to intervene.”

I don't want to derail this thread, so I started another thread to discuss that sentence here.

Originally posted by Brook Gardner-Durbin:

Emilien, I asked because my reaction to this situation was to say this was a legal game state, and the player not announcing their floating mana was not a big problem, and covered by the IPG's sentence "If a minor violation is quickly handled by the players to their mutual satisfaction, a judge does not need to intervene."

Sure, but here:
- the violation isn't handled
- I am pretty sure AP wouldn't be satisfied at all.