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.
Lovlighet:
Standard: Ikke lovlig
Pioneer: Lovlig
Modern: Lovlig
Legacy: Lovlig
Vintage: Lovlig
EDH / Commander: Lovlig
Pauper: Ikke lovlig
Explorer: Lovlig
Historic: Lovlig
Brawl: Ikke lovlig
Premodern: Ikke lovlig
93/94: Ikke lovlig