Here’s a bit of advice for people like me who are really, really good at forgetting things while they’re running an RPG.
Consider the following scenario. The characters are doing X, and you made a note at some point that Y should happen. But then in the rewarding chaos of running a combat encounter, you forget the exact details of how Y works, so you run it wrong. There are lots of different scenarios that this encounter algebra can cover. Y can be the fine details of what an illusion can or can’t do, or misremembering a creature’s resistance to a damage type as immunity, or forgetting that an effect should have ended after just 1 round. It’s a complicated game, and forgetting things is easy.
First off, understand that it’s simple enough to just say to the players, “Sorry, I messed that up. Let’s just assume that Y happened in a different way than what we played out.” Everyone makes mistakes from time to time. Your players understand that. Especially if you’re playing with people you know well, no one’s going to get uptight or hold a grudge over you slipping up. (If they do get uptight or hold a grudge, you might be playing with the wrong people, but that’s another topic.)
Alternatively, though, it’s often possible — and remarkably easy — to roll with your mistakes and figure out ways to cover for them after the fact.
Here’s an example near and dear to me. Every time I run troglodytes in combat (and I sincerely mean: every single time, for the last 40-odd years), I forget to ask for saving throws for their stench at the start of the fight. Every time. And usually when this happens, one of the players points this out in round 2 or 3, and I just say, “Oh, right. Let’s assume you saved earlier and we’ll start rolling now.”
But you can also embrace that mistake whole-heartedly, by saying words to the effect of: “Yeah, that’s unusual. These troglodytes aren’t exuding their normal stench, and you have no idea why.”
Then at some point after the combat, you get to figure out why. Depending on the timing, you might have the entire length of the break between your game sessions to think of something.
Maybe these troglodytes are suffering some magical malady that’s weakening their tribe, putting the characters in a position to do them a favor by discovering the source of the malady and ending it. Maybe they’ve all become cultists of the god of hygiene. Who knows?
Well, you know. Because you can pivot off of that innocuous error to make your the game’s narrative line up with the error any way you like.
Always remember how much of what’s fun in an RPG comes about because of randomness. Players and characters doing things that no one could possibly have predicted. And in the same way, the act of GMs making mistakes — the simple process of forgetting details in the way all GMs do from time to time — can create the same kind of fun by taking the session or the campaign as a whole in a different direction.