BENFICA keeper Odisseas Vlachodimos made a howler against Belenenses after leaving the ball to float into the corner from a 35-yard free-kick.
Benfica were up 2-0 thanks to a Jonas Gonçalves Oliveira strike and Nuno Miguel Prata Coelho own goal.
However, after Belenenses forward Joao Diogo Alves Rodrigues was bundled down on 66 minutes the away side were given a free-kick.
Diogo Viana whipped in a free-kick from 35 yards with the cross missing out his team-mates.
Benfica keeper Vlachodimos confidently left to ball believing it was going wide – only for the free-kick to bounce into the corner.
The howler paved the way for a Belenenses comeback, who gained confidence from the shock error.
Two minutes later Rodrigues, who won the free-kick, equalised in the Estadio da Luz.
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The controversy did not stop there as midfielder Eduardo Henrique was booked for the second time on the 94th minute and sent off.
Belenenses held onto the point as they sit in in seventh place, while Benfica lost ground at the top of the table.
After the 2-2 home draw, Benfica sit top of the league, leading Porto only by goal difference.
Benfica missed out on the chance to go two points clear.