I failed to have the dummy follow suit because the remaining card of that suit was accidentally hidden under a card of another suit. This was discovered later in the game, and treated as a normal revoke. Should the penalty have been reduced because the dummy is face up and everyone (opponents included) failed to notice that it had only 12 visible cards to begin with?
The actual answer is in Law 64D
there is no trick penalty for the established revoke ...3. if the revoke was made in failing to play any card faced on, or belonging to a hand faced on, the
table, including a card from dummy’s hand.
But a clearer view is given in In bridge, is there a penalty if dummy incorrectly lays down his hand? (not quite a duplicate but very close).