Rules earn legitimacy when they survive inconvenient cases.

A rule that only holds when it is convenient has not been tested. Legitimacy is built in the cases where following the standard costs something, where the sympathetic person loses and the unsympathetic one is protected. Those are the moments everyone remembers when deciding later whether the rule means anything.

When an inconvenient case arrives, treat it as the point rather than the exception. Getting it right there buys you more compliance than a hundred easy applications.