Barnstaple were given a reality check by Ivybridge, who defeated the long-time South West Premier Division leaders 17-10.
Going into the game Barum were 14 points clear at the top of the table and already planning for promotion back to National Two South at the end of the season.
Ivybridge had gone nine games without a win and were hovering just below mid-table.
Barnstaple may have been a player or two short of a full-strength side, but Ivybridge could only play and beat what was put in front of them.
Among the players absent from the Barnstaple side was skipper Winston James, whose presence was definitely missed.
"Sometimes you don't realise what the leader does until he is not there to do it," mused Barum's playing chief Jeremy Chugg.
"We had changes in the front row because our hooker was suspended - and we reshuffled the backs because Ryan Carter was out and that meant moving several players to cover him.
"Changes don't help, but we still had enough decent players on the field to have done better than we did."
Barnstaple are still top, and Ivybridge remain mid-table, but for their player-coach Jamie Tripcony it was a result to savour.
"It was a tough game in tough conditions, but we needed that win" said Tripcony, a former Barnstaple player.
"Everyone bought into the game plan and the guys put in one hell of a shift for one another.
"We knew the non-talent work off the ball would win us the game in those conditions and everyone stepped up to that."
Matt Dayment put Ivybridge's first points on the board with a run-in try set up by Alex Gozzard. Flanker Adam Northcott was mauled over for a second try on 32 minutes. Gozzard kicked the conversion for a 12-0 half-time lead.
Barnstaple got a try back from Brendon Darley, which left a score in it with half an hour to go.
Ivybridge hooker Jay Geraty was bundled over by weight of numbers behind him for a third try that gave the home side a two-score cushion.
Barnstaple chipped away at the deficit when No.8 Jake Childs went through on 73 minutes, but time ran out before they could claw back another try.
Barnstaple are away to Bracknell this Saturday in a rearranged game.