Bayern were held in disappointing fashion by the Bundesliga's second-lowest scorers, with a host of crucial stars failing to provide any cutting edge despite Vincent Kompany's side having 80 per cent possession and the majority of the chances.
Having progressed in the Champions League earlier this week at the expense of rivals Bayer Leverkusen, Bayern were brought down to earth by Benedict Hollerbach's 83rd minute equaliser, with the forward slotting in from close range after Jonas Urbig struggled to punch away a cross by Union.
Bayern had felt some relief having been held for the majority of the game by Union before Leroy Sane finished off Josip Stanicic's low cross in the 75th minute in an otherwise low-quality performance from the visitors' attack.
Clear cut chances were few and far between from both sides, with both goalkeepers being forced to thwart numerous efforts across the match, however the game was mostly contested in midfield, with Bayern struggling to break down a stubborn Union defence.
Despite the slip up, Bayern still remain nine points clear at the top of the Bundesliga, although Leverkusen will have an opportunity to close the gap to six when they play later this weekend. However, the frustration has continued to build around Kompany even with the lead and the Champions League progress after Bayern's shock defeat to relegation strugglers Bochum last week.