Rules are generally made deliberately, but I've known more than one game that was made more fun by an accidental misreading of the rules. When a rule mistake is important enough to spoil a game, or breaks the consistency, one generally notices. I guess it's possible for a game to just be silently slightly less fun than it should be because of a rule mistake, but I haven't seen it happen much. Which possibly says something about the value of complex rules with lots of special cases.