(CNN) - It's been a few decades since their heyday, but this isn't something you'd expect to see at a New Kids on the Block concert.
Fans were shocked and disappointed when one of the singers walked off the stage during a concert in New York.
According to onlookers, Jonathan Knight left in the middle of the performance Thursday night, and never returned.
The rest of NKOTB kept the show going, playing a mix of their old hits from the late '80s and early '90s, as fans chanted for Knight's return.
The singer has spoken in the past about his struggle with anxiety and many suspect that's what happened Thursday evening.
Knight used his Twitter account to offer a simple apology to fans, tweeting "I'm sorry."