I'm a British beginner and am learning by playing 2 player Mahjong with my Chinese lady in Hong Kong. Yesterday, she picked up the 9 of circles that I discarded and laid it with 2 other 9s to (so I initially thought) make a Pong. She then laid all her other tiles declaring Mahjong. I pointed out that there was no pair - but instead the 7 and 8 of circles. She moved the 9 to join the 7 and 8 to make a Chow, leaving the the other two 9s as the required pair. She regularly plays 4 player Mahjong and assured me that this is ok. Whilst it was technically a winning hand, I felt that the game should be void as she had mis-declared. COMMENTS PLEASE
Change of Claim: If a player, after announcing a claim for “chi”, “pong” or “kong”, changes his mind and wishes to change his claim into one of another type (or to “win”), this is usually allowed without penalty. (Unless the player has already exposed his set, in which case any illegally exposed tiles become penalty tiles). The player should not do this too frequently, otherwise the judge may impose a penalty.
According to the facts as you have laid them out, the change of claim made by your lady is acceptable.
In regards the parenthesized section above, regarding Penalty Tiles:
"If mahjong were a two-player game, such exposure would be disadvantageous to no one but the offender himself; thus, no penalty would be necessary, and the offender could just retrieve the exposed tiles and continue playing."
The point of Penalty Tiles is to allow a game to continue after an irregularity without undue advantage to any of the other three players not party to the irregularity. As the current deal is ending, there is no need to exact a penalty in order to permit fair continuation of play.
How precisely these things are handled depends a lot on the ruleset used.
The following is from the "Green Book" detailing the Mahjong Competition Rules.
"A player must not declare "Chi", "Pung," or "Kong," followed immediately by the declaration of "Hu." When a discarded tile completes the hand, the player must say "Hu" only. Commission of this error, or errors involving "Hu" declarations in nonstandard ways, or failure to reveal his standing concealed tiles, prohibit his winning on the present turn; he'll have to discard a tile and hope to declare "Hu" on another turn. If a player shows his tiles but neglects to declare "Hu," he forfeits the right to win, and must continue playing.
Revealing one's tiles before declaring "Hu" forfeits the player's right to win the current hand."
BUT: This is from the official rules of the format, describing how play should proceed at high level events, such as international tournaments.
I would advice to not be this strict when playing for fun (and especially when playing a 2-player variant, which will certainly not use this ruleset anyway).