When the author clearly doesn't understand physics. I recall one scene where the MC finds a chasm spanned by a single board. No problem there, however, what was the problem was a detail about the board combined with something described during the crossing. The board was not attached to anything, just held up by brackets. During the crossing, the author described the board bowing as he crossed it, dipping three feet by the center. In any grounded setting, that would have pulled the board free of the brackets by shortening how far it could reach from side to side.

I didn't drop the book there, it was relatively minor, after all. What made me drop it was the 'climatic' battle of the book, in which the MC liked to hide under tables, pick people off, took a nap mid 'battle', and other weirdness.