Former Harlequins lock Billy Davidson's Cobham came off second best as they crashed 15-10 at home to London Division One South leaders Chobham on Saturday.
The Fairmile Lane club had gone into the game third in the table, knowing a bonus point win could have seen them claim top spot.
The result in the end saw them slip back to sixth in table with Chobham stretching out a four point advantage at the summit.
Elsewhere, league newboys Weybridge Vandals found the going tought in a 60-12 at Old Alleynians in London Division two South West, while Thames Ditton's Old Paulines lost 11-7 at Old Blues in Surrey Division One
Cobham host Chichester this week, while Vandals welcome Trojans to Brownacres and Old Paulines travel to Old Wimbledonians.