A player may not use a Joker for either a single tile or in a group of single tiles. This rule also applies to pairs or a group of tiles with pairs. A single tile defined is a stand-alone tile or a group of tiles that are not identical i.e. 2020 or NEWS. During the game, a player may not call to expose a group of single tiles unless the tile called is the winning tile that gives them mah jongg.
Stopping the Charleston after the first Charleston moves is permitted and does not require another players' permission to do so nor does that person need to give a reason as to why they make their decision.