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Something interesting just happened and I've no idea why. I was playing my usual East-Only game at Tenhou. It was rather uneventful - the deal rotated four times rather quickly and after the 4th new dealer hand the game dealt out a bonus hand. The indicator before the deal showed a '1' again. It wasn't the typical dealer bonus hand. At first I thought maybe I had accidentally clicked to play a full game but after the bonus hand the game ended. I'm trying to figure out what might trigger that? I'm sure it's a rule I'm just not familiar with. Any guesses?

Read that big post by xKime in your "Finally" topic again:

Also, do note that these rules generally imply playing with a 西入 or 南入 (shaa-nyuu or nan-nyuu) setting, where the game will not end after the last hand in case a player hasn't gotten over the 30,000 points mark. Tenhou implements this, along with a "sudden death" (サドンデス) rule, meaning that at the time a player gets past the 30,000 points mark in the extra round, the game will end. Furthermore, in tenhou, no second extra round is ever played, meaning that a full game will never go into North round, and a half game will never go into West round.

Ah! Yes, this particular game was uneventful - a few quick outs and fast deal rotation. I'm sure none of us were 30k+

Had it not dealt the bonus hand, it might have been the fastest game I've played online.