I once read a book that seemed to be written by two totally different people. The one part was mostly set in the 1940's, and was about the early career of a Harold Robbins type of writer. This part was actually pretty decent, about a young man inexperienced in both writing and sex, and his learning about both. It was reasonable, and made a lot of sense.
This good part was sandwiched in between parts set in the 1970's, where he is worldly famous and a major stud. So the book starts in the 1970s, has a major flashback to the beginning of his career in the 1940s, and then returns to the 1970s.
Well, the 1970s part was just awful. Who ever wrote this part was inexperienced in both writing and sex. They seemed to think that great sex resulted from acrobatic contortions. "Oh, she's great in bed - she can lay on her chest, bring her legs up over her head and fold herself in half." Seriously, every sexual encounter was a description of the people putting themselves in these weird, convoluted positions because that is great sex!
My theory as to why the book had such a split personality was that someone had written the 1940's part, but it was too short to be published as is (maybe 125 pages), so the second person added the other sections. I personally will never forget the image of the hero's wife bending backwards until her head went through her legs, and how sexy it was supposed to be, and all I could think of was I hope she doesn't get a cramp.