Forest boss Colin Calderwood saw his side lose for the first time in his reign and said: "It is disappointing to lose and be out of the Cup and we did not do enough.
"I ask myself if it was a setback or a catastrophe. I have to say it was a setback because we just did not perform well enough.
"We are doing well in the league but there is always something that comes up and bites you on the backside and it happened tonight."
Accrington boss John Coleman added: "I think we can play better than that.
"We can pass the ball and control the game better, but now I just hope for the club's sake that we have a money-spinning second-round tie."