Upstarts Sassuolo went second in the table and ended Napoli's 100% record on the pitch in Serie A when a Manuel Locatelli penalty and a solo Maxime Lopez goal gave them a shock 2-0 away win on Sunday.
Lopez had to wait nearly two minutes for his stoppage time goal to be confirmed while VAR officials reviewed a possible penalty for Napoli immediately beforehand.
Napoli's Victor Osimhen complained that he had his shirt tugged but play continued, Sassuolo broke upfield and Lopez left two Napoli defenders on the ground before slipping the ball past David Ospina.
VAR officials eventually ruled it was not a penalty and the goal stood amid angry Napoli protests.
Sassuolo had taken the lead after Giovanni Di Lorenzo gave away a penalty with a late tackle on Giacomo Raspadori and Locatelli scored from the spot in the 59th minute.
Dries Mertens should have equalised but somehow shot wide from close range.
Unbeaten Sassuolo have 14 points from six games, two behind leaders AC Milan, while Napoli have 11.
Napoli won their first four games on the pitch but were handed a 3-0 forfeit and had a point deducted after failing to play their match away to Juventus amid of confusion over COVID-19 rules.