Possible Clues:

• Instance that doesn't conform to a rule
• Adverse criticism? Not as a rule
• Bar one performing what's against the rules
• Expect no one but one may appear
• Omission that may be taken as offence
• It may be taken after a slight breach of rule
• Something not conforming to a rule
• Rule thwarter
• It proves the rule, but no one returns
• Objectors may take it as a special case