OK, so the subject line was a bit misleading as you really can’t make a mistake as a Dungeon Master- even the rules are suggestions on how to play. Any “mistake” that you think you have made in an adventure can easily be corrected and used as a role playing opportunity.
That said, as we push to develop our DM skills, there are three action points that we want to avoid as they tend to shut down the player’s enthusiasm and limit the opportunities for role-playing in a game.
As a DM the one rule that I 100% agree with is never say “NO” in a game. If your players want to do something, go with it and allow them to do it, as long as you are up-front at the potential consequences.
You want to play an beast race or half-orc in a civilized game setting, OK, but you are going to draw unwanted attention to yourself and the party. Use that example as a chance at roll playing.
Dice are an important part of D&D- they are the analog logic engine that drives the game though a series of checks and charts in the rules. Use them to create the story, but don’t punish the players with auto-result dice rolls. Always give them a chance or a way out vs. just rolling the dice and saying tough you on the results. The dice should build and keep the suspense, not kill it.
Ending the adventure…
Each gaming session has a set time to play it- a few hours, evening, or even a marathon all day event, but always take stock of where you are in the adventure during the last hour or two of the gaming session.
Sometimes you have to stop before an adventure is finished- find that natural stopping point.
If you can run the big boss battle run it, if you can’t then end the session with it setting up, and said big boss ogre-mage taunting the party…
…with the battle starting next week.