Bayonne edged Harlequins 16-12 in a messy Amlin Challenge Cup encounter as torrential rain put paid to any chance of flowing rugby.
Four Nick Evans penalties put Quins well on top in the first half as only Joe Pietersen's superb try for the home side lifted the gloom.
But a farcical second from Sam Gerber kick-started the fightback and the French side kept their noses in front.
Evans had kicked Quins ahead before Bayonne produced what would prove a rare moment of inspiration in the ninth minute. Full-back Pepito Elhorga defied the conditions with a superb turn of speed to put winger Pietersen over in the corner.
Benjamin Boyet was well short with the conversion attempt and two subsequent penalties as Evans kept his footing better on the treacherous surface to land his four shots - including one via an upright and another from around 50 yards.
The second try arrived five minutes into the second period when Boyet's kick stuck in a huge puddle, taking Quins full-back Mike Brown out of the game and allowing Gerber to splash over in the corner. Cedric Garcia took on kicking duties and found the distance, but his effort struck the post and bounced to safety.
Evans missed for the first time with a penalty from the left touchline before Quins lost captain Chris Robshaw for 10 minutes when he was yellow-carded for a late, high tackle. Scrum-half Garcia showed the way for kickers by cutting his run-up to two paces and slotting over the penalty to open up a four-point advantage.
The visitors had a late spell of pressure but were unable to force over and a shambolic finish saw the ball twice deliberately kicked out of play ahead of the whistle - the second time by Quins number eight Nick Easter, in search of a losing bonus point.
Elsewhere, a late try from flank Wouter Moore gave Italian side I Cavalieri Estra a 23-21 victory over Connacht on Saturday.