Also, whether Tolkien's elves have pointed ears or not.
Pick an event in a Harry Potter book. Any event, major, minor, irrelevant, plot-related, character-related...any event, in any Harry Potter book. It is guaranteed that someone, somewhere, at some point in time, has had a flamewar over something about that event.
One of the most notable events: the epilogue. It divided the fanbase in two; this post, for example, gives logical reasons why the epilogue had some significance but overall was not happy with it; and some people were unhappy because they wanted to decide for themselves what happened after the death of Voldemort, and felt that the epilogue felt like bad fanfiction.
Discussion on any Expanded Universe. On the official Dungeons & Dragons forums, discussing any of the novels is an offense punishable by an instant ban, because of what appears to have been one of the worst flame wars in recorded history. The admins and mods did a VERY thorough job of making sure that the posts comprising the Flame War got deleted, and if a mod even hints at the cause of it, they are fired immediately.
Please, never ask anyone what their favorite Warrior Cats character is, or it will lapse into a never ending debate of Jayfeather vs. Hawkfrost vs. Harry Potter.
Go on the official forum and mention that Cinderheart is the best character ever and not a Mary Sue, or say she is a horrible Mary Sue. Alternately, mention Lionblaze's mate. Hey, they burned down my house!
Henrik Wergeland and rival Johan Sebastian Welhaven managed to start a Flame War with a clipboard and handwritten insults in the 1830s - it got so intense the whole university joined in, chose sides, and when the thing eventually got into the newspapers, it escalated Up to Eleven. As tropes go, the flame war was exaggerated by literates, to last for another 150 years, if not even longer.
Even more awesome when considering the two of them wrote out the entire thing in competing four line verses!