The Elements of Harmony (2) - S1-E2
Other mistake: At the end of the episode when Princess Celestia is talking to Twilight Sparkle you can see that Princess Celestia's tail is on top of her leg instead of in-between. It's covering her cutie mark.
Other mistake: In the scene where Barca is bleeding at the pool, Batiatus comes and grabs his hair. Barca abruptly moves before Batiatus starts to actually pull his hair.
Other mistake: Duke is checking old copies of the Haven Herald around the time of Lucy (27 years ago). A headline on the one he's reading is about Lucy's disappearance. A moment later, he's reading a story about a meteor shower that occurs every 27 years; but the text we can see states that it just happened, and refers to dates in 2010 (having just happened, not '27 years from now'). This paper is supposed to be a 1983 edition. Also there is fake Latin 'filler text' printed in some columns.
Suggested correction: Lucy's disappearance is at the top of the newspaper and the hunter storm is at the bottom of the same page. Also, the paper predicts the next storm to occur. Just like how we know when to expect the next lunar eclipse or the next meteor shower, so I don't see how this is a mistake.
It's not a prediction of events to come, it's an article telling about the recent event, which states it was seen on the morning of the October 21, 2010. It even compares the 2010 shower to a previous shower that occurred in 2009. The mistake is valid.