KIRSTIE ALLEY is on "Dancing With Stars" this season, and she was doing pretty well.  That surprised some people, because of her weight.  (--GEORGE LOPEZ being one of them.)

And that 'surprise' made OTHER people mad, because it's not fair to ASSUME bigger ladies can't shake a leg.

Anyway, last night's episode will probably just make the debate more confusing.  Because about 12 seconds into the beginning of Kirstie's performance, her partner's leg gave out from under her . . . while he was trying to dip her.

Her partner Maksim Chmerkovskiy says his thigh "just gave out" . . . but it's not clear whether it happened because of Kirstie's weight.  He grimaced in pain a few times, and they finished the dance pretty well, with a better dip half a minute later.

Check out the video.