It happened before, f.e. German bundesliga 2001:
Schalke 04 had just won their home game 5:3, when Serjej Barbarez scored the go-ahead goal for HSV in Hamburg vs Bayern Munich in the 89th minute. Schalke already started celebrating the championship (because Schalke & Bayern would have finished with 62pts apiece, but Schalke owned a better goal difference, so they would have won that tie-breaker) because they were misinformed and thought the game in Hamburg already was over with Bayern losing 0-1. Then the game in Hamburg was put on the big screen and Bayern was awarded a free kick inside Hamburgs penalty area in the 93rd minute...and what do you know, they scored the equalizer to take away the title from Schalke:
