I play exclusively on my 5c iPhone. The pieces on the display are very close together. I sometimes move the wrong piece before I realize it. Then I am stuck with a move that no competent player would make. I have seen other players do the same thing. I scratch my head wondering what was with that move? Then I realize that they too would like to UNDO that move. The Game needs an UNDO button/function to allow for these obvious mistakes to be corrected.