The incident happened when the singer attempted to throw the stand across the stage during a performance at Chicago's House Of Blues last week.
Before leaving the stage, she apologised to the audience member, saying: "Didn't mean to hit you there."
Blogging on her Twitter account later, she said: "I was trying to throw it off stage but it went into the crowd. Turns out I'm a bad rock star.
"I did however feel really bad about it, I looked and made sure everyone was okay and had someone go and double check."
Last week, Knowles collapsed at a New York airport after becoming unwell as a result of cold medication.