Members of the studio audience on the ABC show gasped in horror as the former 'Cheers' actress and the professional dancer fell to the floor during their rumba routine to 'Somewhere Over the Rainbow'.
However, the pair carried on with the dance, although Maksim appeared to be limping after his leg gave way.
Afterwards, he said: "My thigh just gave out. I'm so sorry."
Judge Bruno Tonioli commended the couple for completing their routine, despite the hiccup, and achieving a score of 21 out of 30.
He said: "You can't keep good talent down. Once you got up, it was better than ever."
Kirstie, 60, isn't worried the incident could lead to her leaving the show because things like that can happen on live television.
She said: "There's something exciting about live TV. I'm so scared because something can happen and now that it's happened I'm like, 'So what?' "
The theme of the evening was to perform a dance that told a personal story about the contestant.
Model Petra Nemcova and NFL player Hines Ward were the two top-scoring celebrities of the night, finishing in joint first place with 25 points each.
Talk show host Wendy Williams finished in bottom place, scoring a paltry 15 points after she failed to impress the three judges Bruno, Len Goodman and Carrie Ann Inaba with her foxtrot to 'Last Night a DJ Saved My Life'.