Exeter Chiefs' Danny Gray missed a last minute conversion as his side lost by a single point against London Welsh.
The Chiefs started the game strongly, with a Matt Jess try helping them into a 10-3 lead at half-time.
Welsh appeared rejuvenated after the break and took a 16-10 lead, thanks to a a penalty try and two penalties.
With the game in stoppage time, Chiefs winger Mark Foster went over for a try that put them within a point, but Gray was unable to give Exeter the win.
Despite the loss the Chiefs still lead Pool B of the play-offs and must now beat Nottingham in their final game on Saturday, 1 May to make sure of their place in the semi-finals.