It isn't just little things, sometimes it's a major premise. Like the net energy problem mentioned above that would render their work useless except as theoretical research. Like some of the practices of the Catholic Church in Angels and Demons. In his early book Digital Fortress he has somebody invent a totally unbreakable cipher. Okay, I'll suspend disbelief for a novel. But the only copy of the algorithm is encoded using the unbreakable algorithm. Huh? You don't lock the only key to a safe inside the safe. Computers don't burn up from thinking too hard, like in DF. That went out with 20,000 Leagues Under the Sea. And the less glaring discrepancies are so numerous. Search for Dan Brown fact errors, or fact checking Dan Brown for lists of them that readers noticed.
@B93 - I don't like Dan Brown, and I don't think he's a very good writer. But you're kind of making my point, which is that this sort of thing isn't likely to significantly impact sales in most cases because the number of readers who realize and/or care is pretty small. Using Dan Brown as an example doesn't refute my point because he's sold an estimate 200 million books (according to Wikipedia, anyway).
There is always the fictional city that is based upon a real one. Change the street names a bit, use a map of that real city to plot your locations for the action and just write. If your work needs to reference a real place I think it is imperative that you get that real place right, especially if it is a well known city like Paris or New York. Most readers put down a book and find something else to read if they are constantly rolling their eyes about details.
