The X-Men: First Class star has been wowing audiences at the Trafalgar Studios in London since stepping into the role of the doomed Celtic king last month (Feb13), but he admits he ran into a little memory trouble during a show this week.
He tells the Press Association, "I realised that I said the same line twice and I went, 'Oh dear this is a bad moment for me'."
However, MCAvoy appeared to get away with the slip-up - because fans "didn't quite notice".
He adds, "Some of them will have, but I don't think anybody else noticed and then I got it back on track and it was fine."
MCAvoy recently revealed the intense stage show has taken its toll on his body and he has had to undergo regular physiotherapy sessions to treat bumps and bruises sustained from the performances.