For me, it's...
Looking in mirrors.
Strange things happening--strange things that the protagonist is not overly concerned about, but that would freak out a real person.
No driving reason for the book revealed by a (generous) page forty.
Science that isn't based on or spinning off of any actual science.
Specific writing tics, especially involving the construction "Doing this, I did this other thing at the same time (even though it is physically impossible)."
Disembodied body parts. ("Green and blue eyes met across the room. Two heavy feet walked up behind me. A pair of strong biceps fluttered becomingly.")
Series with brave, bold, ambitious plots in book 1, but the rest of the series acts to reverse all of the bad things, rather than asking the protagonist to grow or change or understand.
Teenagers utterly SO WHINY BORED with their comfortable lives.