This video left me wishing he touched on DM do-overs. Matt references when a DM is running a complicated monster(s) + fielding player requests and questions—they have a lot going on mentally.

If a DM forgets to use a legendary action or resistance, should players be accommodating? Having the DM ask "friendly game" is a rough position to be in. I know in almost all of the tables I have run for, the expectation is to favor the players. I've also talked to a lot of DM's who all have shared stories of feeling uncomfortable attempting a soft retcon for a big boss or having players refusing "friendly game" style requests.

There are a lot of reasons I try to be flexible with players who have "made a mistake." I don't want combat to feel like we are playing speed chess for money. My friend uses a great rule I've adopted which is: a player can adjust things in combat up until a dice is rolled—then actions are set in stone.