This is true mainly for the avatar and Harry Potter fandoms.
A lot more than those. This is a partial list just off the top of my head:
Several other ships in Harry Potter, besides RH vs. HH
Star Trek
Star Wars EU
Sherlock Holmes (including Sherlock)
Doctor Who
X-Files
Buffy the Vampire Slayer
Twilight
Hunger Games
Xena Warrior Princess
Friends
House
Merlin
Vampire Diaries
Artemis Fowl
Arrow
Big Bang Theory
Pirates of the Caribbean
Personally I think this was the most foolish thing she ever did giving the commotion it caused.
At the risk of repeating myself on this subject, Rowling only made herself look ridiculous by mentioning it after seven years. It doesn't change what she wrote, how she wrote it, or what R/H fans new all along -- that R/H was endgame. It's one thing to personally prefer H/Hr for whatever reason, it's another thing entirely to argue that it was obvious or even hinted at in the book. Having Rowling say it might have been a mistake, personality-wise, to put Ron and Hermione together in no way vindicates readers who claimed H/Hr was plain as day in the plot.
It would be very similar if, seven years from now, Kishimoto were to say, "You know, it might not have been the best idea to put Naruto and Sakura together. They might have been happier with other people." NH fans would crow about it, as if they were right all along, but all that counts is what the author writes IN THE STORY.
I seriously doubt Kishimoto would say something that absurd later on, but I never thought Rowling would, either. 
*rubs temples and chants, "Calm blue ocean..."*