What do you think are some of the most common mistakes Dungeon Masters make during a Role-playing Game?
One of the most common I see is a lack of flexibility. People have a plan and they try to stick to it. But players can and will botch that plan in a heartbeat. The mistake made is many will go too far in trying to force things back to fit the plan. If all you have to do is nudge a bit, fine. But if your rewriting things, bringing killed characters back, and making nonsensical rulings.
IMO, one of the biggest mistakes Dungeon Masters make is letting players control the plot of the game completely. When someone is Dungeon Master of a group, they should have a general plot in mind and guide the players along that plot as best as possible. Some junior Dungeon Masters seem to be just there to run the monsters and provide the treasure - much like any melee only arcade game. This isn't much fun for anyone but hack and slash players, and will typically lose anyone not into that kind of simplistic game.
The second common mistake I see occur is Monty Haul campaigns, where the treasure rewards far outweigh the challenges presented. The likelihood of a tribe of orcs having a +3 sword is zero, since any slightly more powerful critter encountering them would have taken such a thing, long before the players run into them.
The third biggest error a Dungeon Master can make is to bias games unfairly, either by making the players have no options during the campaign, or making the challenges nearly impossible for players to overcome, without warning of some kind.
Again these are all IMO, so take with bags of salt, as you wish.
First big mistake… they don't know the rules of the game well enough. There is nothing worst than seeing a player correcting a Dungeon Master on a move or their character's ability unless home brew rules were presented beforehand and agreed to.
Plus 1 to what JB said specially since that happened to me a couple of times already. Another I will add is dragging out a scene or event so much that it becomes like a board game because you see the same pieces, pictures and actions every time.