A Cardiff win would have been their fifth on the trot and made it their best run in the competition since 2010, but they surprisingly came unstuck 13-7 against a Treviso side who had lost all 13 of their matches this term.
In fact, the last time that Treviso had tasted Guinness PRO12 success was on February 20, 2015, when they beat Cardiff on home soil.
And the Italian side proved to have Cardiff's measure once again at the Stadio Comunale di Monigo, running out victors thanks to a penalty try and eight points from the boot of Jayden Hayward.
In reply, the Blues were only able to get on the scoresheet through Blaine Scully's first-half try and the conversion from Rhys Patchell.
The United States international crossed in the 34th minute following an overlap and that helped Cardiff overturn two Hayward penalties to take a 7-6 lead at half-time.
However, a penalty try conceded by the visitors seven minutes into the second half for pulling down the scrum proved decisive, as ninth-placed Cardiff could find no way back after that.
That try, along with the conversion from Hayward, were the only points scored by either side in the second half as Treviso held on for a long-awaited victory following 20 successive defeats.