Yes - she stated that Grindelwald was dead, and he wasn’t.
In an interview in 2005, two years before the final book, J.K. Rowling says that Grindelwald is dead.
ES: Our other “Ask Jo” question (the one about James and Lily’s sacrifices), was from Maria Vlasiou, who is 25, of the Netherlands. And then the third is from Helen Poole, 18, from Thirsk, Yorkshire – also one of the “Plot Thickens” fan book authors. It’s the one about Grindelwald, which I’m sure you’ve been gearing up for us to ask.
JKR: Uh huh.
ES: Clearly -
JKR: Come on then, remind me. Is he dead?
ES: Yeah, is he dead?
JKR: Yeah, he is.
- Leaky Cauldron interview (16 July, 2005)
However, in the seventh book, he’s shown to be alive, until the Dark Lord kills him. In this case, she was wrong about something that wasn’t published yet. (I’ll look for other examples and update).
She said Brazil won the World Cup 5 times - no Latin team did yet.
In Quidditch Through the Ages, it’s mentioned that Peru is likely to be the first Latin American country to win the Quidditch World Cup, meaning none had won yet when this was published.
“Argentina and Brazil both reached the quarter-finals of the World Cup in the last century. Undoubtedly the most skilled Quidditch nation in South America is Peru, which is tipped to become the first Latin World Cup winner within ten years.”
- Quidditch Through the Ages
Quidditch Through the Ages is up to date to at least 1994, since it mentions a game played that year, so it can’t possibly have been written and not updated with new content since before 1994.
“The exception to this general rule is Japan, where Quidditch has been gaining steadily in popularity over the last century. The most successful Japanese team, the Toyohashi Tengu, narrowly missed a win over Lithuania’s Gorodok Gargoyles in 1994.”
- Quidditch Through the Ages
However, in the Quidditch World Cup writings J.K. Rowling published on Pottermore, she says the Brazilian team won the Quidditch World Cup five times, while also saying that the time that could theoretically be accounted for by Quidditch Through the Ages being published before it happened, since it’s at least as current as 1994, was when they weren’t playing well and having bad years.
Brazil has won the Quidditch World Cup five times, but the nineties and early noughties were generally considered wilderness years for this once great side. Manager José Barboza has reinvigorated the national game, bringing in younger players from every corner of the country. With an average age of only 22, this is the least experienced side remaining in the tournament.
- Quidditch World Cup 2014: Daily Prophet Reports (Pottermore)
In the Pottermore writing on Quirrell, she contradicts the timing.
In “Harry Potter and the Philosopher’s Stone”, Quirrell was possessed by the Dark Lord after he failed to get the Stone from Gringotts, so the Dark Lord decided to watch him more closely.
“Since then, I have served him faithfully, although I have let him down many times. He has had to be very hard on me.’ Quirrell shivered suddenly. ‘He does not forgive mistakes easily. When I failed to steal the Stone from Gringotts, he was most displeased. He punished me … decided he would have to keep a closer watch on me …”
- Harry Potter and the Philosopher's Stone, Chapter 17 (The Man with Two Faces)
However, in the Pottermore writing, J.K. Rowling says that the Dark Lord possesses Quirrell immediately upon encountering him and realizing he taught at Hogwarts, instead of after he was already serving the Dark Lord but failed to get him the Philosopher’s Stone.
Though Hagrid was correct in saying that Quirrell had a ‘brilliant mind’, the Hogwarts teacher was both naive and arrogant in thinking that he would be able to control an encounter with Voldemort, even in the Dark wizard’s weakened state. When Voldemort realised that the young man had a position at Hogwarts, he took immediate possession of Quirrell, who was incapable of resisting.
- Professor Quirrell (Pottermore)