Jayson Tatum's appearance at Drake's concert on Wednesday night turned out to be a very painful experience for one fan ... 'cause during her freak-out over the Celtics star's cameo -- she wiped out down six rows of stairs.
The wild incident happened in the nosebleeds at TD Garden arena during the It's All A Blur Tour's stop in Boston ... after the rapper introduced the NBA star to the sold-out crowd.
The woman started to jump up and down when Tatum got on stage -- and she screamed at the top of her lungs, "Jayson Tatum!