As a GM, you need to know something if you don’t already: you’re going to screw up. You’re going to make mistakes, you’re going to try and cover them up, you’re going to hem and haw and you’re going to make things up that make no sense whatsoever. Get used to that idea; and while you don't necessarily need to be content with it, become comfortable with the idea that you're not infallible, and become comfortable that you don't have to be.
That's part of the problem with a policy of "the GM is always right": eventually, we start believing our own lie. The problem with believing the lie, of course, is that we forget it's a lie, and we forget why we started telling it: so that gameplay doesn't get bogged down with minutia.
That's precisely the kind of pitfall a GM can fall into. Let's take a look at some of the more common ones:
Tyranny: The classic role of the GM as the final decision-maker can very easily lead to a slippery slope wherein the GM can start to see himself as the only person at the table whose opinion matters. GMs who suffer from this delusion don't have players for very long. Even GMs who only speak like they're always right, but only do this to show they "just have to be convinced" they're wrong, will find themselves painted by players with this brush. The reason this is such a pitfall is because the players and the GMs together make a game happen; if players don't feel like they're going to be heard, if they feel they are value-less to a GM, they will not want to game with that GM. The easiest way to avoid this trap is through talking with your players, and listening to them, and being ready to make compromises. But watch out! If you talk with your players and listen to them, but never compromise or never side with them, you're only confirming your tyranny.
Just Doing Your Job: The "Jobber"-style GM is often GMing because somebody needs to, not because he particularly cares or is enthusiastic about the job. If you’re not enthusiastic about GMing—well, you’re probably not reading this article, frankly, nor should you be GMing. But if you are enthusiastic, make sure that you don’t just sit behind your screen, read the box text at the players, and roll dice. Really take the opportunity to fire up their imaginations and your own, set the scene with creativity and flair. Doing so will enliven the game and make it more enjoyable for everyone—even you! It will also help prevent a proliferation of players who just do the same. Because, really, how fun is mumbling at your shirt and then rolling some dice? It’s supposed to be fun, so have fun with it!
Catering to Players: The opposite of tyranny is bending over backward to guarantee your players are getting what they want. The classic Twilight Zone episode "A Nice Place to Visit" highlights why this is such a bad thing: if you give your players everything they think they want, they will eventually feel satiated, then over-satiated. They will have too much of a good thing, without necessarily having earned it. This can be treasure, this can be success, this can be anything representative of benefit without proportionate challenge. Players, almost universally, like challenge.
Not Catering to Players: Of course, the amount of challenge your players prefer is going to be different with every group of players you run. And if you find you're providing more challenge than the proportionate rewards, you'll hear about it. People like to gain benefits—or at the very least, not suffer detriments—for the effort they put forth; that's the core mechanic of the real-world economic system, many real-world systems of morality, and every game system. If you find your players making joking comments like "I'm doing my entertainment wrong," you may be overdoing the challenge a bit; roll it back a notch.
Not Keeping It Balanced: The GM is in a lot of ways a tight-rope walker, and sometimes you'll find yourself overdoing something in one direction or another. The amount you cater to your players is a good example; others include how much storytelling and social tactics you include versus combat, and how often the enemies know victory versus the PCs and vice versa. This balance is different from group to group, table to table and session to session; you must try to be aware of when player interest is flagging or when you have provided too much of one thing and too little of another.
Destinies: Having player characters who have destinies is a very interesting tactic, if you have the ability to control the future, your players' lives, and time itself. Many GMs like to say that the characters in their games are destined for greatness, and some take it so far to have some kind of prophecy about what will happen to that character. But there are a couple of problems inherent in this idea: first, that means the players' characters are never in any actual danger. Bob the Mighty could fight the dragon or sit down and have a nap during his attack; he is in equal amounts of danger, if his character is destined for some greatness or will not die here because of some prophecy about him. This leads directly into the biggest problem with destinies: unless very carefully crafted, they limit your narrative flexibility as a GM and can be very easily manipulated by clever players. But, if your "destined for greatness" simply means that a character won't die because the dice are on an uncooperative streak, or won't be killed so long as he doesn't behave stupidly. . .that's also not really a destiny in the classical sense. The idea behind avoiding these is to always keep your options open—players might leave the campaign, or (Heavens forbid) get hit by a bus, or you might simply run out of time to tell the story. Destinies make these situations more difficult to deal with.
Sacrificing Yourself: Many GMs can go overboard, building so much into their sessions and their worlds that they have no time for any pursuits other than gaming. This can lead to temporary or even permanent GM burnout. Eric has previously warned us against burning ourselves out in this excellent article. Our hobby needs to keep its good GMs, so if you feel like your hobby is becoming a job, take a step back, enjoy some other things for a while. This might mean a sabbatical from GMing for a few weeks or months, it might mean taking time as a player instead of a GM, it might even mean just making sure to watch all your favorite shows and taking some time for non-game-related pastimes every week. Just don’t lose sight of the fact that this is ultimately supposed to be fun!
An Adversarial Relationship: This is the big one to avoid and, for many GMs, the hardest to avoid. It's worth repeating: the players winning does not require the GM losing. Many GMs, new and seasoned alike, take on an adversarial tone with their players, actively attempting to kill them or cause them to fail in their goals, even to the extent of invoking Rule Zero ("The GM makes the rules, and the GM's decision is final") to modify the rules to make it more probable and/or guaranteed. This is, flat-out, player abuse. This is the kind of behavior that births a lack of trust between players and GMs, and this is the kind of behavior that creates Rules Lawyers. Many players hate that GMs have control over them because those players have some point in their past encountered GMs who behave this way. By behaving in this way, you are not only hurting your players and the likelihood they will return to your table; you are making the life of other GMs more difficult, and you are shrinking the hobby. Because many players will look upon this kind of bullying and say, "I don't need to be so maltreated by my entertainment." The GM who plays this way has forgotten that roleplaying games are for the enjoyment of everyone at the table, not just himself.
There are other pitfalls that you will encounter as you GM, and the pitfalls are different from table to table, system to system, player to player. The list above just scratches the surface, but they are good things to keep in mind. There are even pitfalls that haven't been mentioned here, things that are a part of any decent person's moral system: don't deliberately offend them, don't strike them, don't confuse reality with fantasy. It will help you to think to yourself, "were the tables turned, and were I on the other side of this Screen, would I be enjoying myself?" If not, then you must absolutely reconsider your actions.
As we wind down this series, we will ponder exactly what impact GMs have in the real world: do we make the world a better place, even if it's just one person, or one night, at a time? Or is that pompous and over-reaching? We'll take a look at this idea in our conclusion. See you then!