It's not that unusual for flares to be lit in the stands during football matches, especially when a team scores, and especially on mainland Europe.
In 2005, AC Milan goalkeeper Dida was hit by a flare which was thrown by an Inter Milan fan during the Champions League Quarter Final second leg clash between the two same-city rivals. Inter fans threw several flares and bottles onto the pitch, with one of flares hitting Dida on the shoulder. The match was abandoned and Uefa awarded AC Milan a 3-0 aggregate win. Inter Milan were fined £132,000 and ordered to play their next four games behind closed doors.