There are a number of reasons besides the marker having a

(1) shot on wrong target
(2) shot on next target deflects off the frame of target into next impact zone
(3) unstable soil in impact zone falling due to sound of adjacent shot.
Now what we dont have as markers and butts officers in the rules is a clear procedure for returning the target to the firing position once the error has been discovered without causing a disadvantage to an F Class shooter who used his previous shot as a reference for correcting his next.
Some times I have seen the target returned CLEAN and sometimes with the spotter in place but no value disk and sometimes with both spotter and value disk in place and that can confuse the scorer and shooter alike.
If there is a rule please point it out to me.