Cornish Pirates were undone by London Welsh at the Mennaye Field on Saturday, despite leading 24-6 at half-time.
Converted tries from Nick Jackson (2) and Aisea Havilli along with a Rob Cook penalty had put the Pirates firmly in control at the break.
Welsh then fought back with second-half tries from Nathan Bonner-Evans, Charlie Gower and Ben Thomas.
Ed Lewis-Pratt and Gordon Ross added the extras with the boot to steal the win for the Exiles.
The Pirates now start the second-phase of the Championship season with an away tie at Bristol next Friday in Pool A of the play-offs.
Welsh start their play-off campaign with home game against Doncaster Knights in Pool B.