The singer appeared to suffer an anxiety attack while performing at the iHeartRadio Theater in New York and abruptly left the stage as he prepared to sing the ballad Survive You, leaving his bandmates Donnie Wahlberg, Joey MCIntyre, Danny Wood and brother Jordan Knight to continue with the hour-long set without him.
They joked to fans that Knight, who has battled bouts of anxiety and panic attacks in the past, had to answer a call of nature and was stuck in the restroom, and he subsequently took to his Twitter.com blog to apologise to devotees for his odd behaviour.
He simply wrote, "I'm sorry..." while Wahlberg attempted to clear the air by posting, "To recap - Joe was especially dazzling! Jordan was especially amazing! Danny was especially solid & Jon was especially shy!"
The five-piece put the problematic gig behind them as they appeared on the Today show on Friday for an interview and performance, and although they did not mention Thursday's walk-off on air, MCIntyre logged on to his Twitter page after the programme to express his pride at Knight's return.
He writes, "So proud of @JonathanRKnight for coming back strong on the @TodayShow this morning."