I’m wrong a lot. Like, a lot. It’s a big part of the reason my first campaigns were so incredibly linear, because I couldn’t reconcile being the all-powerful expert running the game with being the guy who thought his rubber pot lid was oven-safe. I thought the DM had to be right all the time, and that included knowing how the rules worked and where the plot was going and whether using this monster was a good idea and so forth. I still kind of feel this way: Law #1 is “The DM is always right”.
But the first corollary is “But not always entirely right”. In this case, that means the DM is right, but not necessarily right now. Tabletop gaming is a storytelling medium that lets you tell stories over time, and that gives you a chance to revise earlier decisions in a way self-contained media can’t. A DM can say anything he or she wants, but also has the power to change their mind later and issue a retcon if that makes the game better.
Retcon is short for “retroactive continuity”. It’s the act of making a change to a story such that an event that already occurred instead occurred differently, and everything else in the story uses that change instead of the original version. Rather than belabor the point with further examples, I’ll just link you to TV Tropes, though I also recommend the full version of the manga page in the trope picture.
In D&D and other tabletop games, retconning is easy: the DM tells the players something happened a certain way, and that’s how it happened. It’s the equivalent of a text box in a comic book that says “From now on Superman was raised in Queens — Ed.” It very obviously breaks the fourth wall, but it doesn’t matter if it’s more heavy-handed than in other media. It’s partially because of the direct relationship between the creator (DM) and consumers (players) that other media doesn’t have, but also because the consumers are themselves creators. A player can say “I’ve decided my character is a blonde”, and everybody just accepts it without several essays of justification and a court order. Movies can’t do that, and if you disagree, please consider researching one James Bond.
The most important part about retconning something in a game isn’t how to do it, but when. Obviously it’s a terrible idea to retcon away a player’s moment of glory or something that ended up being incredibly fun. But a lot of systems also track character resources, and the more resources there are, the harder it is to back up to a time before they were expended. Fate systems may only need you to remember a few stress tracks or fate points, but in D&D you have to remember hit points, cast spells, used powers, consumed items, spent ammunition, and the like, on top of forgetting all the story changes. This is why it’s generally a bad idea to retcon a fight; too much goes into the result to change the result on the fly.* It takes a very specific kind of player or software to remember every action that occurred in a fight and set things back to how they were beforehand. The same may be true of a long skill challenge, or even exploration or interaction encounters. After all, suspense and emotional investment are resources you can spend, just ones you can’t track numerically or roll back.
There’s one exception to this: retconning a TPK. I completely understand if a party decides a particular fight was objectively bad and letting it stand ruins the campaign entirely. I usually feel there are story-based ways to get around it, to let the failure stand and roll into something else, but in some situations that just isn’t possible. Here a retcon still isn’t a good idea, but it may be the best idea among bad ones.
The DMing books I’ve read actually do recommending using a retcon for one specific case: rules questions. There’s an oft-cited example of the player who tries to do something ludicrous and it’s not immediately clear whether the rules allow or even cover it. Consider the player who wants to summon an elephant ten feet above the wizard. A DM may not know whether this is legal, and finding the section in the rules that says “you can’t summon a creature into an environment that can’t support it” can take forever, especially in the days of paper books without search functions. The books recommend the DM make a decision and use it until he has a chance to verify, usually between sessions, then use the correct rules. This is sort of a retcon in that technically things didn’t happen that way and the players are free to invent another reason the wizard suddenly took two hundred damage and became flat, but again, nobody is recommending re-running the fight without that specific action.
My retcons tend to be story-based, and for me the “when” is “whenever the decision I made affects how the game will play out for the worse.” Sometimes it’s small, like when I changed the caliphate in my current campaign to a sultanate. Between sessions one of my players pointed out how strongly caliphates were aligned to a religion, and I didn’t want that, so I changed it. Except to an especially historically, religiously, or etymologically aligned player both words mean basically the same thing, so the context shift wasn’t a problem.
The other big one we did was when the players suffered a planar travel mishap and ended up on Carceri, the plane of prisons. Carceri was the result of a random roll at the end of the session, and as we were packing up I tried to think about what I would do with Carceri next week. The answer was, unfortunately, nothing. There was no fun idea I had related to the campaign plot, and doing a side quest there would derail things just as they were starting to pick up. Instead I told the players on their way out the door that they had actually landed on Utopia, the plane of city. They had allies there, it was relatively non-hostile, and there was any chance something would happen that could lead into this campaign or another one later. By all metrics it was a better landing spot, so that’s what happened. At the next session the players faffed about on Utopia, purchased some perfectly cubic pastries, and went on their way without problem. I didn’t have to scramble for a meaningful plot, the players got to do what they wanted without random interference, and we got everything back on track much faster than we would have if we had adhered to the will of the dice.
The point of both is that they made the game better, one because it helped the plot and the other because it helped our impression of the world. That’s ostensibly the point of the retcon: something wasn’t working, so you fix it. It’s a delicate task to decide what’s worth changing and what isn’t. Most parties might decide changing “caliphate” to “sultanate” isn’t worth learning how to pronounce a second word, or they like the feel of the word better regardless of the religious connotations, or they like the connotations and want to adopt them. They can decide what they want to do with a ninety-second conversation. Bigger changes require more consideration and perhaps table-wide agreement, but the point is that it’s possible. You’re not beholden to your past decisions any more than the players are, and it’s well within your rights to change anything from an NPC’s name to the entire grappling ruleset if it’s affecting how you feel about the game.
I guess my suggestion is the same as always: talk to your players if you’re going to change anything major. There’s a chance any change that enhances your game will hurt theirs and there has to be a balance, if not a third option that makes everybody happy. But for small changes, you’re probably fine. Your players aren’t likely to judge you harshly if an NPC’s race or accent changes. The ability to adjust continuity like that is one of the advantages of this type of media, and it would be a shame to ignore it when it’s needed.
* — This said, I kind of like the idea of a campaign where the battles are intentionally ludicrously hard, where the players have to try them again and again until they succeed. Maybe something like Edge of Tomorrow where the players are stuck in a time loop, or something that mimics a video game where the players take snapshots of their characters at “save points”. But since repeating fights is intentional, those aren’t really retcons, and that’s beyond the scope of this article.