The defeat saw Dortmund's dreams of winning the title this season in tatters as they lost the chance of going top of the Bundesliga table. Dortmund who are currently second on the table with 69 points, one behind leaders Bayern Munich were forced to finish the match with 9 men as Mario Gotze and Marius Wolf were shown red cards.
Sancho was hit in the face by the lighter as they celebrated in front of the home fans but was able to continue.
Munich now only need to win three of their final four games - starting with Sunday's trip to Nuremberg, to win a seventh consecutive Bundesliga title. The Ruhr derby had started off so well for Dortmund - who were nine points clear in December and top until 6 April when they lost 5-0 to Bayern.