Quick answer
No, a discard cannot be taken back once it is named or placed on the table. When two players call the same tile, a call to win beats a call for an exposure. A joker may never be passed in the Charleston, and that includes the courtesy pass. You hold thirteen tiles, or fourteen on your turn before you discard, with exposed sets counted in the total.
Some table arguments recur because the rules are silent or split on them, and others because players learned different house conventions. Here is where the League's rules land on each one, and where your table has to agree on its own.
Can you take a discard back?
No. A tile is discarded once it is named or placed on the table, and either one is enough. This one is not ambiguous.
That also settles whether you can change your mind while the tile is still in your fingers. You can, until you name it or it touches the table, whichever comes first. Once you have said the tile, it is gone even if your hand is still on it.
Two people call the same tile
Under League rules, a call to complete a winning hand beats a call for an exposure.
When two players call for the same thing, both for an exposure or both for mahjong, the tile goes to the player next in turn from the discarder, the one sitting closer to the discarder's right. It is not a race, so speaking first does not win it.
There is one exception. Once a caller has started to expose tiles for the claim, it is too late for the player next in turn to speak up, and the tile stays with the caller who is exposing.
A call and a rack at the same moment
A discard can be called until the next player racks the tile they drew or discards. Now and then the call and the rack happen at the same moment, and nobody can say which came first.
The standardised tournament rules settle it in the caller's favour. When a tile is claimed at the same time as the next player racks or discards, the caller gets the benefit of the doubt, and the tournament director gives the official ruling. The same rules ask players to pause for a second between picking a tile and racking it, which gives the others a fair chance to call. A home table has no director, so agree before the first hand whether a close call goes to the caller.
Can you pass a joker in the Charleston?
No. A joker may never be passed in the Charleston, and that includes the optional courtesy pass at the end. The courtesy pass is part of the Charleston, so the no-jokers rule applies to it the same way.
Receiving one is against the rules as well. If a joker comes to you in a pass, give it straight back and say so.
How many tiles should I have?
Thirteen, except when it is your turn and you have picked or claimed a tile but not yet discarded, when you hold fourteen. East starts the hand holding fourteen and discards first. Exposed sets count toward your total: if you have exposed a pung, you hold ten in the rack plus those three.
Every call moves tiles from your rack to the exposures, so the habit worth building is counting after every call rather than only when something feels wrong.
Do you have to lay out a winning hand in card order?
Under the standardised tournament rules, yes. A declared hand should be laid out as it appears on the card, and if you lay it out in another order and another player asks you to arrange it that way, you must. A player who refuses can have the hand declared dead, and the game continues.
The rule is about the declared hand. During play, your exposures can sit on top of the rack in any order. At home, laying a winning hand out in card order also makes it quicker for the table to check against the card.
Do we have to play the current card?
Yes, if you want to play the standard game. The National Mah Jongg League issues a new card each April and the hands change, so an expired card describes hands that no longer count.
A group can agree to keep playing last year's card among themselves, for example while waiting for new cards to arrive. Just make sure everyone at the table is on the same one.
The meta-rule
For anything genuinely ambiguous, the answer is: agree before the hand, not during it. When two people learned different house conventions, the middle of a hand is the worst time to find that out.
A new player asking 'how does your table handle this' at the start of a session is being courteous, not naive.
Put it into practice
New to the game? Start the course at Lesson 1, Meet the Tiles. The first three lessons are free and need no account.
Already play? Have the coach read a real dealt hand for you, free. It names the direction the hand goes for, what to keep, what to pass, and the reasoning behind each call.
A rules argument can start mid-hand, when somebody is not sure what they are building and exposes something that does not fit. Our AI coach reads a dealt hand and names the direction it goes for, free and with no account.