Darwin Nunez scored in the 99th minute for Liverpool's 1-0 win - but only after the game had been stopped for a head injury suffered by Ibrahima Konate.
When the game restarted, Liverpool were given the ball, even though Forest had possession when referee Paul Tierney had stopped play.
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Nuno said: "I don't want to talk about the referee, sorry.
"We are frustrated, we are disappointed, we are sad. We did a good job and the City Ground, as always, was amazing for us. The disappointment is not giving the supporters what they deserve."
Similar to Man Utd game? "We have chances, we don't score and then we get punished at the end."
On the performance, he added: "It was good. We created moments and clear situations. The boys did really well. We are sad, it will take a while."