Juventus' unbeaten start to the season ended with a 4-2 loss at Sassuolo after Wojciech Szczesny's goalkeeping errors contributed to their defeat in Serie A on Saturday.
The Polish international suffered a howler after 12 minutes when Armand Lauriente's long-range shot deflected off Szczesny's gloves and into the net.
Then eight minutes from time Szczesny inadvertently parried a rebound into the path of Andrea Pinamont, who headed home to make it 3-2 to Sassuolo.
The nightmare was complete for Juve five minutes into stoppage time when defender Federico Gatti, seemingly unaware Szczesny was off his line, tapped the ball into his own goal.
Juventus had first equalised after 21 minutes when Sassuolo defender Matias Vina inadvertently poked the ball into his own net from close range while attempting to clear a cross.
The hosts reclaimed the lead four minutes before halftime through Domenico Berardi before Federico Chiesa levelled or the visitors in the 78th minute with a deflected shot.
The defeat leaves Juve in fourth place on 10 points from five games, two points behind leaders Inter Milan who visit Empoli on Sunday. Sassuolo are 11th on six points.