I was running Scourge of the Slave Lords back during the days of yore. Given, I'd only been DMing for a dozen or so years at the time, so I was clearly a novice prepared to make a novice mistake... *grins*
There was a segment at the slavers' mountain stockade when the party proceeded into the upper story of the "curtain wall" structure and encountered a hobgoblin guard contingent with their chief, who was calmly feeding cheese to his crow... which I somehow read as "cow" in the description. And once I'd said it, despite how ridiculous it sounded, I stubbornly stuck with... particularly after scanning the paragraph several times and repeatedly coming up with the word "cow".
After the session, which frankly became somewhat ludicrous in the wake of my little faux pas, since the players just couldn't get over the cow, I finally discovered my error when the magical "r" appeared in the creature's description.
I became a tad bit more discriminating after that regarding my perspective on and presentation of boxed text... not to mention my preparation techniques.