I've certainly made my share—like thinking casting Heath Ledger as Joker was questionable at best, thinking Robocop sounded like it was going to be a really dumb movie (the title sounded SO dumb to me when I first started seeing the commercials), and The Lego Movie? How can that be good?
But my biggest had to be before Titanic came out. “It's the most expensive film ever made (at the time)” I said. “Even if it's a big hit it will never make its money back.”