When I was in high school, I read this mystery novel where some evil man discovered Beethoven’s 10th Symphony, so he hired a bunch of symphonians to perform it. When the performance was over he was going to whack them all, then destroy the score so he could be the only person in history who’d ever heard the symphony performed. A totally believable premise, I know. But I was willing to play along because I was nuts about music, and hey! Beethoven! 

But then I came across this scene where the conductor was in rehearsals with the orchestra and he said, “Bass clarinet, louder.”
And I – an insanely nerdy bass clarinet player – was like “FOOL! BASS CLARINETS DIDN’T EXIST UNTIL WAGNER’S TIME. Which also means your so-called 10th Symphony is totally fake!” Then I flung the book out the window and scared the cat.
I’ll give you a Götterfunken right between the eyes, you terrible writer!
If the book had been any good, I might have put up with this little mistake. But there were a number of other details the author didn’t bother to think through – like this guy got murdered because someone jabbed a long needle into his lungs and he fell over and drowned in his own blood! And I was like, “How is that even possible? That’s like being murdered by paper cuts.”
Have you run across instances where the author fell off the turnip truck? In a writerly sense, of course.