I was playing a concealed hand, but mistakenly called a discard to make an exposure. What can I do?
The answer depends on where you are in the process of claiming that discard. If you have only called the discard, or have picked it up but still have it in your hand, you can change your mind by saying something along the lines of 'sorry, I don't need that after all'.
However, if the discard was placed on the top part of your rack, or if you exposed any of the tiles from your hand on the top part of your rack before picking up the discard, the tile is now yours.
If you find that you are committed to keeping the discarded tile, then you must now move to playing a non-concealed hand. You'll need to quickly assess your options and make adjustments as appropriate. For example, if you expose a kong of 7 bams and you'd have more options if you changed this to a pung, then do so prior to discarding and ending your turn. Alternatively, you may add jokers as appropriate to make a kong or quint prior to discarding.