We are going question by question through the National Mah Jongg League’s newsletter Q&A.
Here’s the question from the League:
Q. During a Joker exchange, Player B erroneously put an 8 crak onto Player A's rack. Player A noticed the error before discarding and replaced the incorrect 8 crak with the proper tile, a 3 crak. Is this legal?
A. If Player A had noticed the error immediately, she could have returned the 8 crak to Player B and put the Joker back in its place. However, once Player B discarded, Player A had an erroneous exposure and her hand should have been declared dead, with no penalty to Player B. You may never exchange a symbol tile for another symbol tile, and Joker exchanges should be done hand-to-hand over the table.
Here is another reason that it is important to pay attention when people are making joker exchanges, especially if you’re the person with the exposed joker. 
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