Bob Taylor's response in MMD 040101 is right on.  Putting pianos on
their sides is okay if everything is tight.  The only player that
I know of that can have problems is the newer Aeolian style players
with the stack under the keybed.  This is the stack with the plastic
valves.

When the piano is placed on its side, the stack can slip down to one
side and become misaligned with the keys.  What happens then is the
player will strike two keys for each hole in the paper or the keys will
seem to stick and not play properly.  The simple solution is to loosen
the nuts holding the stack to the keybed and slide it back into place
until the piano is playing properly.  Then tighten the screws and the
job is done.

This could conceivably happen to any player but I have had to work on
two of the newer Aeolian's that had this very problem.

Pete Knobloch (Tempe Arizona, USA)