If the user has made multiple 5 of a Kinds, and all other players have
finished, and the user clicks the "Undo" button to select a different
score slot, after selecting the new score slot the program hangs with
"Computer playing for Alice" in the title bar.

I suspect, but cannot confirm, that the user must have enough extra 5 of
a Kinds so that all other players are finished before the start of the
turn where the user clicks "Undo", and the user must also have another
turn after that.

If this would be a high score, which is significantly more likely than
if the player had not made multiple 5 of a Kinds, the high score is not
recorded (obviously, because the game is not finished).

I have attached a screenshot.

...Marvin