At the point where 'Nikki' is found in the bathtub by Ron, she is hauled out of the tub fully clothed and soaked - but in the next shot on the bathtub edge, her shirt (underneath the towel given to dry her) is visibly dry and gets wet again as she attempts to drown him.
When Ron sees Nikki in the bathtub and rushes over to pull her out, it is obvious that his jeans are already wet, despite the fact that he hasn't even reached the water yet.
When Susan gets her hand caught in the garbage disposal, a shot from behind shows an obvious stunt double for Karen Black: the double having much thicker legs and ankles.