Bayern Munich came looking for a title party and were served with a cold dose of reality as survival scrappers Mainz earned a stunning 2-1 win over the Bundesliga leaders.
First-half goals from Jonathan Burkardt and Robin Quaison put in-form Mainz in charge, and a leggy Bayern side - who would have been crowned champions with a victory - lacked the zip in their game to overturn that deficit.
Manuel Neuer was at fault for the opener, but also produced some big saves to keep Bayern in touch with their impressive hosts, who have been the most-improved team in the league in the second half of the season.
When Quaison headed the second, he showed the aerial threat that was rarely seen from Bayern's fit-again talisman Robert Lewandowski in this game. Lewandowski snatched a last-gasp consolation after a defensive clanger, but it came too late to be significant.
Burkardt stunned Bayern in the third minute when he was left unattended on the edge of the penalty area and fired a thumping shot that Neuer could only help into the left corner.
Bayern had Lewandowski back in their ranks after a knee injury absence, but before the Polish striker could show any threat they were almost two goals behind, Neuer producing a fine save at full stretch to touch a 20-yard header from Danny Latza against the left post.
Lewandowski fired waywardly from his first chance before Neuer again prevented Bayern slipping two goals behind, Quaison turning Jerome Boateng far too easily and hitting the target with a thumping shot that bounced away off the well-placed goalkeeper.
Bo Svensson's Mainz side, having won four and drawn two of their previous six games to surge out of the relegation zone, doubled their lead in the 37th minute when Quaison headed past Neuer after Philipp Mwene's free-kick from the left proved a perfect, teasing delivery.
Hansi Flick's reaction was to bring on three substitutes at the start of the second half, hauling off Leon Goretzka, who was perhaps fortunate to avoid a first-half red card, plus Kingsley Coman and Leroy Sane, and introducing Eric Maxim Choupo-Moting, Jamal Musiala and Tanguy Nianzou.
A stray arm from Lewandowski caught Mainz midfielder Leandro Barreiro in the face, drawing blood, but it appeared accidental. It was nevertheless not the impact Bayern were looking for from their star striker.
Bayern trailed 2-0 in the first meeting of these sides this season but recovered to win 5-2; however, until Lewandowski's stoppage-time strike, their second-half goal threat in this game was limited to half-chances that the likes of Choupo-Moting and Joshua Kimmich could not convert.