Leaders Bayer Leverkusen defeated 10-man Union Berlin 1-0 on Saturday and could claim their first-ever Bundesliga title next weekend at home against Werder Bremen.
Florian Wirtz scored the winner shortly before the break to give Leverkusen a 16-point advantage over second-placed Bayern Munich, who were up 2-0 but conceded a shocking 3-2 defeat to promoted Heindeinhem.
"My plan was (to win the title) a bit later. The situation is very good for us now and maybe we can celebrate something on Sunday. But before that there is the Europa League and that is where all the focus lies," coach Xabi Alonso said in a news conference.
Leverkusen, who are unbeaten in all their 41 season matches and a record-equalling 28 in the Bundesliga in 2023-24, will can be crowned champions if they beat Bremen on Sunday.
But they could even take the title the previous day if Bayern and VfB Stuttgart, who won 1-0 at Borussia Dortmund in the late game to draw level with Bayern, lose their matches against Cologne and Eintracht Frankfurt, respectively.
"We should all be a bit ashamed of ourselves today. And we should make sure that we represent the Bayern crest in a more dignified way relatively quickly," Bayern board member for sport Max Eberl said.
Leverkusen are in the quarter-finals of the Europa League against West Ham and play the first leg on Wednesday. They have also secured a spot in the Cup final.
Leverkusen have been in the Bundesliga since 1979, and won the UEFA Cup in 1998 and German Cup in 1993. But they have not lifted a trophy since then, with several near misses including three lost Cup finals and five Bundesliga runner-up finishes - which earned them the nickname Vizekusen (Vicekusen) in Germany and Neverkusen abroad.
Elsewhere, RB Leipzig thrashed Freiburg 4-1, Mainz defeated Darmstadt 4-0 and Cologne scored two goals in stoppage time to claim a 2-1 comeback win against Bochum.
In Dortmund, Bundesliga number two scorer Serhou Guirassy found the breakthrough for Stuttgart in the 64th with a side-foot effort into the top corner, scoring his 24th goal this season for a crucial victory.
"It's a decent gap," Stuttgart midfielder Angelo Stiller said of the seven-point gap to Dortmund. "But we know there are still six games to go."
Alejandro Grimaldo in the 10th minute and Borja Iglesias in the 15th were both denied by Union goalkeeper Frederik Rönnow as Leverkusen were yet to have a clear goal chance.
In the third minute of added time, Union defender Robin Gosens was sent off after a second yellow card for his foul on Nathan Tella.
After the resulting free-kick, Piero Hincapie's shot was nudged onto the post by Christopher Trimmel's arm, but Odilon Kossounou found the net. The goal was ruled out for offside, but Leverkusen were still awarded a penalty due to Trimmel's offence and Wirtz hit the net from the spot to give Leverkusen the win.
The league leaders celebrated the return from injury of top scorer Victor Boniface, who played his first league game in 108 days.
Leverkusen equalled Bayern's record from 2013-14 of 28 unbeaten matches in one Bundesliga season
"We have to realize that we can talk about the title starting today. Because we can really make history. It's a dream for a lot of people," midfielder Granit Xhaka said.
In Heidenheim, Thomas Müller made his 700th game for Bayern and only former goalkeeper Sepp Maier has more appearances for the club with 709.
The Bavarians looked set to give Müller a win in his special game when they finally broke the deadlock in the 38th. Serge Gnabry lifted the ball for the onrushing Harry Kane to break Heidenheim's resistance.
Gnabry got himself a goal before the break, completing a cross from Alphonso Davies and increasing Bayern's lead.
But the collapse of the German record champions started early in the second half as Heidenheim scored two goals in less than two minutes.
Bayern were caught out by a long ball from goalkeeper Kevin Müller and substitute Kevin Sessa went past Dayot Upamecano and through on goal for a finish in the 50th.
The guests were again caught off guard by another long ball and this time Tim Kleindienst was there to hit the net in the 51st.
Keeper Müller denied Kane in the 56th and the English striker lashed across the goal from just inside the area in the 69th, before Heindenheim started getting dangerous again.
Sessa's header in the 72nd was stopped by Ulreich, while Jan-Niklas Beste's free-kick sailed over the crossbar in the 75th, but Heidenheim managed to stun Bayern in the 79th as Kleindienst completed his brace.
"Everyone involved realises that we're not in the best period in the club's history," Müller said. "We had just as many golden opportunities as Heidenheim, but in the end they turned it round and we didn't score any more goals."
