But that's not what I'm talking about. I'm talking about goblins using their bonus action, orcs getting to close in on enemies, a phase spider phasing in. Or maybe I thought it would be more cinematic to have the PCs see the bad guy slip through a door just ahead of them only to have the challenge of "How did he do that? Did he have a readied action? If he did, he can't lock the door behind him!" In reality, the bad guy had plenty of movement and an action; playing by the rules he moved/closed the door/locked it on his initiative. But having them see him getting away was more fun and also told them where he went.
Once in a blue moon, I really, really, obviously screw up. Nobody's perfect. But if it's not life or death or a really, really important let's deal with it after the game. I'm always open to discussion, just not in game.
A lot of these things are minor, nit-picky things. It's not that I'm granting fire immunity to something that should only have fire resistance, it's that their questioning why the dire wolf the goblin is riding has advantage.
This has nothing to do with humility or accepting that I do not know all the rules, or never make mistakes. It's about interrupting the flow of the game for the DM who is juggling the scene, the monster's actions, what's going to happen because the party just did X when you were certain they were going to do Y. It's about interrupting a story being built by the entire group for a rules discussion and disrupting the flow of the game.
I have absolutely no problem calling for advice as a DM if I don't remember a rule. If I seem confused or unsure, mumbling "how do I do" to myself (well mumbling to myself more than normal) that's all good.
But if it's minor, if it's just "how did that goblin have a chance to make a stealth check after he fired an arrow at me", then we can discuss it later. If I really implemented something wrong, I'll fix it in the future. This is also a question of frequency. Does it happen once every 5 or 10 game sessions? Not a big deal. Every session? Multiple times per session? Then we have a problem.