The worst mistake I have made was in 1998 I somehow forgot to remove a seven ball that was used to level the slate. Yes I stretched the bed cloth over it.....that table never played correctly after that.
This one was a joke, but the general thing to avoid is ANYTHING getting between the cloth and the slate or the rails. The last table mechanic I had left a little tiny piece of wood between the cloth and the rails. It wouldn't have affected play, but I would have felt it bridging there. No good. Had to remove the rail, the cloth, the bit of wood and redo. Triple check the slate and the cloth before you put it down. Rub your hand over the entire surface to check for debris. The pros do this little cloth sweep back and forth maneuver. Just check it out carefully before you start stapling (or gluing).