Used to be rules
Going back to the archives of rules:
If an object is moved accidently, by your cue, on a shot, and it rolls more than 6 inches, it was ruled a foul. If less than 6 inches, your opponent had the option of putting it back.
Now, same situation, if accidently moving an object ball, and it also changes the position or path of other object balls, or the cue ball, it is ruled a foul.
The current rules eliminated the 6 inch rule, and your opponent has the option to put it back or leave it there, providing it did not change positon or path of other object balls, or the cue ball. So basically, if your accident involves 2 or more balls, it is a foul, if just one, your opponent has the option to put it back or leave it.