Exception that proves the rule
A desperation tactic used in an argument or debate whereby proof of the speaker's wrongness is somehow held up by the speaker as proof of his rightness.
Manny: I utterly hate homosexuals. They are an affront to God and man. Except for Rick Astley. I could go down on THAT guy all night! I'd same-sex marry him in a heartbeat.
Moe: I thought you said you hated homosexuals?
Manny: Well, he's the exception that proves the rule.
Moe: I thought you said you hated homosexuals?
Manny: Well, he's the exception that proves the rule.