One of the major changes in the 6th Edition Laws is that the opponent now has the option of whether the balls are replaced or stay where they lie following a fault.  A further complication is in bisque play when, should a bisque be taken, the balls must be replaced.

"Law 28(b). [Faults] REMEDIES

2. The striker must ask the adversary whether he wishes the fault to be rectified. If the adversary elects rectification, the balls are replaced in accordance with Law 22(d). Otherwise the balls remain or are replaced in the positions they occupied after the first stroke in error (but see Law 37(h) for handicap play). "

The first thing the understand is that this option is only available for faults as defined in Law 28.

• It does not apply, for instance, if a ball involved in a croquet stroke leaves the lawn (Law 20c).
• Similarly it does not apply if the striker has played a wrong ball, even if he committed a fault in the same stroke since if more than one error occurs in the same stroke, it is deemed that only the first of the applicable laws in Laws 25 to 28 applies (Law 24a1)

Players must break the habit of touching balls after a fault until the opponent has made his decision.

For example: the striker has a jump shot through a hoop refereed.  The jump shot goes badly wrong - a double tap (28a8), combind with lawn damage (28a15) and prolonged contact (28a7)! The ball however struggles through the hoop and ends up on the non-playing side.  At this juncture the opponent is asked whether he wishes the ball to be returned to its initial position or remain where it lies.  If the striker forgets this, the referee should remind him.

If it is a handicap match and the striker decides to take a bisque then the ball must be returned to the initial position:

"Law 37h.  If the striker decides to play a bisque after committing a fault that is discovered within the limit of claims, Law 28(b)(2) does not apply and the fault must be rectified."

Given that the striker can decline to take bisque right until the last moment, the final positions of the balls after the fault need to remain marked until the bisque is taken.