Lewis Grabban and Anthony Knockaert failed to make the most of golden chances to break the deadlock.
Cauley Woodrow might also have snatched all three points for Barnsley, but was denied by a fine save from Brice Samba who kept out his close range header.
But it was the performance of referee Leigh Doughty that dominated Ismael’s post-match thoughts.
The Frenchman was angered that his side were not given a penalty when Michal Helik appeared to have been pulled back in the box by Joe Worrall.
“It was a really, really big mistake and a game changer. We need to have a clear and proper decision, so it is a big frustration. We are frustrated by the point, even if we saw a lot of positives,” he said.
“I do not want to speak about all the small decisions in the game, because that can happen. But such a big, big mistake... I cannot explain what has happened. It was clear.
“Everyone saw the same situation. We are not stupid.
“It was a clear penalty for us. It is unacceptable, the performance of the referee. You have to do something, you have to say something. We are fighting to win, which is why it is such a big frustration.
“The penalty could have been a game changer for us. It was a good clean sheet and we will take the point.
“In the second half, we had a really good chance to score and Samba made a great save. But the big frustration was about the referee’s performance. I did not speak to the referee. But this was a game changer for us.”
Forest manager Chris Hughton said: “It was a tough game against a very direct team with a lot of physicality. Firstly, you have to match that.
“You have to give us credit for defending well. Barnsley have players who can hurt you from distance and you have to be consistent in your defending to cope with that.
“We had clear chances to win the game.”
Nottingham Forest: Samba, Christie, Tobias Figueiredo (Mbe Soh 90), Worrall, Bong, Cafu, Sow (Bachirou 58), Knockaert (Mighten 87), Krovinovic, Ameobi, Grabban. Unused substitutes: Smith, Freeman, Jenkinson, Lolley, Blackett, Taylor.
Barnsley: Collins, Sibbick (Sollbauer 64), Helik, Andersen, Brittain, Kane (Jordan Williams 87), Mowatt, Styles, Frieser (Adeboyejo 60), Woodrow, Chaplin (Morris 60). Unused substitutes: Walton, Schmidt, Oduor, Miller, Omar.
Referee: L Doughty (Lancashire).
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