Championship promotion hopefuls Exeter Chiefs suffered their third consecutive defeat as London Welsh won with an injury-time try at Old Deer Park.
After dominating for much of the first half, the Chiefs only had a Saul Nelson try to show for a 5-0 half-time lead.
The home side drew level with a Greg Evans' try after the break, before Matt Jess scored to put Exeter 10-5 up.
But Welsh fought back again and, in injury time, Guy Mercer touched down for a converted try to snatch victory.
Saturday's defeat means that second-placed Exeter now trail leaders Bristol by nine points following their main rivals' 31-15 bonus-points win over Cornish Pirates on Friday.