Terzic's woes deepen as Dortmund held by Mainz
Dortmund impressed early and grabbed the lead thanks to a brilliant free-kick from Julian Brandt but Mainz hit back late in the first half, Sepp van den Berg heading in from close range.
USA forward Gio Reyna had the ball in the net in the final minute for Dortmund but the goal was struck off for offside.
Despite qualifying first in a Champions League group including Paris Saint-Germain, AC Milan and Newcastle, Dortmund have now won just one of their past eight league matches.
Dortmund hosted Mainz for the first time since May, when the home side could only manage a 2-2 draw, which allowed Bayern Munich to move past them and claim the Bundesliga title.
Terzic said Monday the collapse "played no role at all" in the side's preparation for the match and his team started well, overrunning a Mainz side who have won just once all season.
Mainz somehow held on amid an early Dortmund barrage, with English teenager Jamie Bynoe-Gittens hitting the crossbar and blasting wide.
Dortmund eventually broke through after half an hour, Brandt sending a dipping free-kick past Mainz goalkeeper Daniel Batz.
The one-way traffic continued until just before the break, Mainz equalising through Liverpool loanee Van den Berg, who headed in a rebound from a corner.
Dortmund 'keeper Gregor Kobel got a paw to the header but replays showed the ball was inches over the line, with Mainz scoring their first goal since November.
The home side dominated possession in an increasingly tense second half but could not find a winner, while Mainz carved out a handful of half-chances on the counter.
Dortmund finished the match with 10 men as Sebastien Haller picked up an injury to his ankle after Terzic had made all of his five changes.
Elsewhere, promoted Darmstadt came from behind three times at to draw 3-3 at Hoffenheim.
Ihlas Bebou scored two goals and won a penalty which was converted by Andrej Kramaric and looked to have Hoffenheim on course for victory, but Darmstadt's Tim Skarke got his second with five minutes remaining to rescue a point.
Leipzig held
A stunning second-half strike from Justin Njinmah secured Werder Bremen a valuable point in a 1-1 draw at home to RB Leipzig.
With Leipzig dominating the game and leading after a Lois Openda goal, Njinmah shed two defenders and blasted a long-range effort into the net from outside the box with 15 minutes remaining.
"Every point is beautiful, especially against such a strong team," said Bremen forward Leonardo Bittencourt. "And call me crazy, but we should have won."
Bremen, promoted in 2022 after a year in the second division, have now opened up a six-point gap on the bottom four in what is likely to be a season-long relegation scrap.
Leipzig bid farewell to long-time servant Emil Forsberg, who played his last match for the club before moving to New York Red Bulls after nine years in Saxony.
"I'm a bit ticked off," Forsberg told Sky. "I would have loved to win."
Bremen's draw was the first time the 2004 Bundesliga winners have taken a point off any of the top seven teams this season.
Leipzig missed a chance to draw level on points with champions Bayern Munich, who have two games in hand.
Bayern face former coach Niko Kovac's Wolfsburg on Wednesday, while leaders Bayer Leverkusen are at home to Bochum.