Referee Guido Winkmann, who'd allowed play to continue, then blew for half-time and was on his way off the pitch when notified by video referee Bibiana Steinhaus, who was watching in Cologne, that he should check a replay.
Mainz fans protest the Monday night contest by throwing toilet paper onto the field.
The players of Mainz 05 and Freiburg were ordered back onto the pitch having gone to the changing room at halftime because the video assistant referee (VAR) awarded a penalty for a handball in an eventful Bundesliga game on Monday.
The penalty was given for a handball by Marc-Oliver Kempf that the VAR noticed after the halftime whistle had blown. Some were looking at TVs to determine exactly what was going on.
He ordered both sets of players back onto the pitch so that the penalty could be taken.
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De Blasis, who'd had to wait for Alexander Schwolow to re-take his place in goal, held his nerve to score inside the left corner as the Freiburg goalkeeper dived the other way.
"It leaves a bitter taste, but we have to accept it with a heavy heart", Freiburg sporting director Jochen Saier told Eurosport at half-time.
The players then returned to their dressing rooms, however the fans weren't quite done as Mainz supporters suddenly started hurling toilet paper onto the pitch, meaning the second-half was delayed even further as the stewards were left with an almighty clean up.
The 2-0 win sees Mainz leapfrog Freiburg in the Bundesliga table on goal difference, climbing out of the relegation zone at the expense of their defeated opponents.
When the game was eventually restarted Mainz doubled their lead in the 78th minute after a shocking error from the visiting keeper, who gifted the hoists the second and final goal of the game.