The Snow Ball Game (1985)
San Francisco traveled to an icy Mile High Stadium to play the Broncos in 1985. Denver took the lead in the snowy conditions but the Niners had the chance to level with a field goal. Amazingly, the kick never happened because of a ludicrous moment that changed the game (via Bleacher Report).
A fan threw a snowball and caused the holder, Matt Cavanaugh, to fumble the ball. The play dissipated and San Francisco lost the game. Later, the NFL admitted that the referee made a mistake because it was blatant fan interference. It was one of the most ridiculous decisions of the season.