In my case the biggest turn-offs are factual errors.
For instance, I read all three books in the Aaron Tucker mysteries by Jeffrey Cohen. I enjoyed the first book, so I started the second. At one point a fairly conclusive piece of evidence was produced which I knew to be absolutely impossible.
That killed my interest in the story. I kept reading, but never really enjoyed it.
Fortunately I gave the third book a chance. I really liked it.
I'm currently reading a book that I'd describe as a lightweight mix of mystery and comedy -- very light reading, but fun.
At one point a criminal masks his face with a red stocking. After the theft the thief stuffs the red stocking into a back pocket leaving some of it hanging out like a flag. During his escape, he winds up in a cattle pen with a bull. The bull chases the thief because, as everyone knows, bulls hate the color red. -- like the red capes used by bullfighters. Hilarity ensues.
Except, of course, bulls are colorblind. They're attracted to the cape by its movement.
Anyway, in a more serious book this would have killed my interest. In a piece of lightweight entertainment I gave it a pass.