Bridgend handed Cardiff only their second home defeat in 55 league matches at Arms Park with a 24-19 win that was clinched by Craig Warlow's injury time dropped goal.
Swansea were the only team to win at Cardiff in the league last season, but the reigning Welsh-Scottish champions were out-thought and out-fought by a Bridgend side, with half-backs Huw Harries and Warlow pulling the strings.
In driving rain on a sodden pitch, handling errors and turnovers were the dominant features of a dismal first period, with the opening score not coming until the half-hour mark.
Appropriately, it owed a lot to the conditions. Right-wing Mike Peters, a former rugby league player with Warrington and making his Bridgend debut, slid over in the corner despite being tackled yards short by opposite number Craig Morgan, and Warlow converted.
But Morgan led the Cardiff reply, trotting over unopposed to round off constant pressure in which flanker Martyn Williams was outstanding. Neil Jenkins kicked both conversion and a penalty to give Cardiff a 10-7 interval lead.
The Wales fly-half, having passed 350 points for his club this season, slotted over another penalty to open the second half.
But Bridgend hit back immediately when a high kick rebounded off Rhys Williams' shoulder into the arms of former Cardiff favourite Jamie Ringer, who strolled in for Warlow to improve.
A third Jenkins penalty put the home side back in front, but with the wind dropping Bridgend took the lead once more with a converted try by substitute Maama Molitika.
Jenkins' boot cut the gap, but the visitors were not to be denied and it was man-of-the match Warlow who clinched a valuable win which was no doubt cheered by current league leaders Swansea, too.