Because damage remains marked on a creature until it's removed as the turn ends, nonlethal damage dealt to a Spirit you control may become lethal if Supreme Phantom leaves the battlefield during that turn.
Legalitet:
Standard: Ej legal
Pioneer: Legal
Modern: Legal
Legacy: Legal
Vintage: Legal
EDH / Commander: Legal
Pauper: Ej legal
Explorer: Legal
Historic: Legal
Brawl: Ej legal
Premodern: Ej legal
93/94: Ej legal