<< I guess I was just looking for some reassurance that it's not as bad as I'm imagining it. <<

I've heard of this practice but I'd never think of doing it. If you purposely make a mistake in an "unimportant" part of the book, nobody'll ever quote it anyway, so what did it accomplish other than make you (the author) look like a screw-up? And if you put a mistake in an "important" passage, you REALLY look like a screw-up!