Wait, stop. Go back
I used to be very strict about what happened at the table when playing RPGs.
Oh, you didn't specifically mention you were whispering after you sneaked all the way here? The guards attack you.
I forgot a very specific rule that has never come up before? Let me spend ten to fifteen minutes hunting for it. You guys just do whatever, this is important.
There's a reason people keep coming back to my tables: I'm not insane - anymore. One thing is teaching players about consequences, another is working against them at every step. I started out as a pedantic fastidious GM, but those tiny victories "against" players didn't feel good at all. I could see their faces drop when they realised that one tiny misstep meant catastrophic consequences. It might be okay every once in a while, but playing games is supposed to be fun, not defusing-a-bomb-every-five-minutes tense.
And I won't spend any time arguing about rulings over rules, because better writers have done so extensively.
Retcon
Being on the players' side sometimes means you need to interrupt the game, go back and readjust. It seems like a waste of time, especially if what you're suddenly deleting is an hour of play. But!
- Have I enjoyed playing until now, even though it won't become part of our history as a group?
- Do I feel better knowing this last part won't have happened?
- Is it just an easy way out because I don't like the consequences of my choices?
If the answers are YES, YES, NO, then go ahead and retcon. Retcon the whole session, who cares! That wasn't wasted time. You still spent time playing. And you deserve to enjoy a game that you play as a hobby. It feels obvious when spelled out, but a lot of groups are very reluctant to retconning anything. The dice are rolled, the numbers on the sheet have been erased and penciled in again (!), so that's done. The end. We all go home feeling bad about what happened, but it obviously can't be changed, can it?
There's a fine line to walk, however, and it's spelled out in question #3. Everyone likes winning, so if you're cheering on the players you might inadvertently end up being too nice just to make them do more, get more, win more. That makes for very lame storytelling.
"Too nice"
Making characters feel good isn't the objective of a good GM. Making players feel good is. It is fundamentally different: players come away happy from the table not because of the biggest loot or the powerful level-up, but because they felt appropriately challenged.
If you put an impossible obstacle in front of players, and they manage to use their own brains or their characters' resources to overcome it, then you can be sure they will leave the table raving about what a good session that was. If no obstacles are ever challenging, or if they are easily bypassed by the GM being moved by the players' struggle, then you aren't telling an interesting story. You aren't playing an RPG. You're playing pretend.
My experience
In my last session, we had two different retcons. That is, the amount of actual consequential play was about half an hour for the whole night.
The first one happened because a player didn't have enough information about a game mechanic. Invisible Sun is a huge complex game, with rules scattered across three different books. Sometimes Quite often things slip. The player tought something was possible, didn't double check the plan with me or the manual, and then after thirty minutes or so of preparation they found out their idea couldn't have worked. We scrapped all that and the group made a new plan.
The second one happened because of me. I was very tired and I was forgetting rules here and there. A player declared their action and I said that wasn't possible because I couldn't remember any rules that allowed it. Spoiler: there were and it was. The consequences for that were incredibly harsh for the whole group, as in "We absolutely don't want this to happen and the whole plan hinged upon this" harsh. Had I been slightly less tired, I would have ruled that that action made sense, and that I would check after the session. But I just said no, make a new plan. The last hour didn't happen.
Was it horrible? Not really. It was a good learning moment for all of us, and most importantly we had fun. At least up until the moment(s) I had to go "Wait, stop. Go back". That session will of course be remembered in eternity as "the retcon session", and we laughed about it quite a lot.
Will I do it again? Retconning, yes. Deleting almost a whole session worth of game, I surely hope not.
I did promise something to my players, though. No more going back because someone (me included) doesn't know how something works. We will discuss rulings together as a group if we don't want to interrupt the game by checking the manuals. But if you don't know the rules or the specifics of an ability of yours and you hate how things turn out, sorry not sorry.
Ignorantia iuris non excusat.
What a fastidious GM.