Referee Wolfgang Stark admitted he made an error by ruling out a valid goal by Borussia Dortmund's Robert Kovac on Sunday.
The Croatian's header clearly crossed the goalline before Hannover goalkeeper Robert Enke fished it out, but Stark did not give the goal.
That would have made the game 2-0 in the home side's favour, but the match eventually ended 1-1.
"We set our standards very high, so that makes this even more bitter," Stark admitted. "(Dortmund keeper) Roman Weidenfeller and the coach (Jurgen) Klopp both lifted our spirits after the game and told us these things can happen now and then."
Nevertheless, the decision cost Dortmund second place in the Bundesliga, and Kovac a rare goal.
"It is particularly annoying because, at 2-0 up, the game would probably have gone differently," said the veteran defender. "And for me it is annoying because I don't score that many goals."
It was the second disappointment in just three days for Dortmund, who were knocked out of the UEFA Cup on a penalty shoot-out against Udinese on Thursday.
But Klopp said: "We started the game excellently and scored a great goal - and even a great second goal, which was unfortunately disallowed.
"At the end, we had two more chances and would therefore have deserved to win."
The draw leaves the Westphalians in sixth place, level on points with local rivals Schalke and Bayer Leverkusen and four points behind league leaders Hamburg.
Their next fixture in a fortnight takes them to Werder Bremen, who are one point behind them in seventh place.