This one is difficult to post a screenshot of considering the placement, which is why I'm posting a wireframe.
I can post a proper screenshot if requested, but it would likely be considered NSFW.
When you adjust the Thigh Tone (pack_leg3_Thigh Tone.ccslider) slider from Ultimate Morphs in either direction, a single vertex on the center line of the body near the beginning of the butt cheeks is moved unintentionally.
When Thigh Tone is negative, it causes a spike. When it is positive, it causes a hole.