The Platinum Rule
How I Met Your Mother Season 3 Episode 11 (12/10/07)
Ted asks the dermatologist removing his butterfly tattoo on a date. The gang tries to talk Ted out of going out with someone he's going to see regularly (aka The Platinum Rule). Everyone shares their horror stories: Robin dating a co-worker, Barney dating Wendy the waitress, and Marshall and Lily double-dating with their neighbors. Ted ends up going for it and getting turned down anyway when she says it's unethical for her to date patients.