This one is more for the players than the gamemasters, especially players who have been playing with the same group for a long time.
Some crusty old Roman (Lucius Apuleius) wrote "parit enim conversatio contemptum", translated as "familiarity breeds contempt." He was comparing gods and kings, so it makes sense to apply the same idiom to the twin nature of the gamemaster as the all-powerful creator and controller of the the gaming world, and the guy you've watched spill food down his front for the last 10+ years.
People change. Mistakes are made, lessons are learned, priorities and styles evolve. It is understandable that occasionally choices made and actions undertaken during an expermental period, especially (frankly) stupid choices and actions, will cause some bruised feelings and subvocal -- and not so subvocal -- mutterings among the players. It's normal, unavoidable, and if not necessarily beneficial, at least not lastingly damaging to the game.
RPG gamers are smart. We are creative. We have to be, because at least 95% of the game is in our minds. We also have good memories, which is essential to game continuity as well as remembering movie lines to use as battle cries. Along with those admirable mental qualities can come other, less desirable social quirks like the ability to hold a grudge indefinitely That's where the danger of Reel 9 comes in.
Let's say the hypothetical GM hypothetically decides to start a campaign with a controversial twist, like all users of magic are hunted by the emperor's armies and being found with a magic item is a death sentence. Done well, this is a setup with great potential. But say this GM discovers after a few sessions that running this particular scenario is not as much fun as he thought, and the players don't seem to be enjoying themselves. So they scrap the whole thing and move on to other adventures. Happens to everyone, water under the bridge, life is a box of chocolates.
One player, though, has an issue. Maybe he had a magic item that he used in public and it was confiscated and destroyed, maybe he threw a spell in a bar fight and got thrown in jail, maybe he couldn't play the character he wanted...whatever the reason, he's not happy. If he can let it go and move forward, no problem. If not...Reel 9.
Reel 9 is the perpetual, repetitive bringing up of a past gaming-related event whenever a similar (or not even similar) situation comes up in a current game. In the above example, in a current game completely unrelated to the original, the GM mentions the comparative rarity of magical healing available in a remote part of the countryside. The formerly-wronged player initiates Reel 9, reminding everyone how his sorceror's wand of cure light wounds was snapped in half by the imperial guards 12 years ago, that the GM hates magic and sorcerors and puppies, and that there's no point in going on if nothing ever changes....
Sound familiar? If not, congratulations. The experience feels like what I imagine a legislative session debating an unresolvable issue would be like if I could stay awake through one. The same rhetoric, the same issues, the same WORDS used again and again. The worst part is the effect it can have on the GM, both overtly and behind the scenes. Outwardly, the GM can basically either ignore it or get angry, because these are not issues can ever be talked out and resolved. They happened years ago, you might as well call Belfast and tell them all's well and there's no more need for hard feelings. What can happen behind the screen, though, is more damaging.
There's a thing called the chilling effect, which I haven't heard talked about in a while, but which was a big deal a while ago. It involves using the threat of some type of unpleasantness (usually a lawsuit) to control the actions of a person or group. It's an insidious form of censorship, nothing else. If one or more players is consistently playing Reel 9 every time the GM tries something new, guess what is going to happen? Either the GM stops innovating and growing, or the player mysteriously stops being told when and where the games are being played. That's a lose-lose scenario, if you're keeping score.
So here's what I suggest: if you're playing Reel 9, knock it off. If your buddy is doing it, help the GM out by changing the subject or distracting the Reel 9er long enough for the GM to stop planning his character's death and get on with the game. If you're the GM, take a deep breath. Leave the table if you must. Maybe they'll get the hint and stop, at least for the moment.
Remember, this is your friend running the game, not some idiot on the other end of your Modern Warfare deathmatch. Your friend who deserves your help and support, if not your respect. Respect would be better, but Apuleius had a point.