It was recently announced The 20th Anniversary Editions of Harry Potter books accidentally added some fake facts to the Ravenclaw and Hufflepuff versions.
In these two versions, it states "Did you know? Professor Sprout had a long-term relationship with fellow teacher Professor Flitwick. Sadly it didn't work out but they remain friends."
The source for this came from as JKR tweet... but it didn't! CollegeHumor, a satirical site known for its entertaining but fake stories, made up this fake tweet for a fake interview-article with JKR titled "J.K. Rowling Knows Way Too Much About Her Characters." This article mixed up real tweets with photoshopped ones.
This isn't the only error to have been discovered and corrected for future reprinting; it was also pointed out that the Ravenclaw edition also lists Luna as having Ollivander’s wand.
What other errors have you heard of or uncovered yourself in printed materials?