Earlier today I posted an entry about pitfalls to avoid when GMing. I noted the common ones known to everyone (fudging dice rolls, railroading, etc.), but I asked others to chime in with their own entries on 3 other common pitfalls to avoid when GMing. So, here is my take on 3 pitfalls to avoid when running a RPG.
1. You didn't make the style of the game clear to the players
You've spent time organizing an adventure, getting the players together, and selecting a nice place to meet up. Only you forgot to mention the theme/style of the game, or even ask the players' input at all. Frequent character death may be a common meme in old school games, but it's not common to everyone who plays RPGs. If your games run a high risk of character death, you need to make this clear up front. Players get annoyed enough when characters die, especially due to bad dice rolls, but it's even more annoying when a player spends several hours working up this amazingly awesome backstory, and fleshing their character out, only to have him/her die against the first batch of giant spiders you throw at the party. Again, frequent character death is almost unheard of in new school RPGs, and if you want to run an old school style game, you need to make it abundantly clear from the beginning. This doesn't just go for frequent character death either. That's just one example. If your game focuses on a mystery the PCs must solve, a combat light horror scenario, or a deathtrap dungeon, you need to at least make some of this known to your players before the game.
2. You plan on using a pet NPC
Avoid this like the plague. There will be moments where you'll think, "hey this is set in that same world I ran my 15th level half-dragon assassin character! Wouldn't it be cool if I introduced him as a NPC in this game?" The simple answer is, no, it wouldn't. Chances are, you'll end up wasting precious time reminiscing about how your uber character knocked off that lich in one round (hyperbole), or managed to con that dragon out of his horde that one time, and frankly, your players don't give a shit. They also don't care to see your pet NPC essentially take over the session doing awesome shit, because you want to show how much of a badass he is. Pet NPCs=bad idea 100% of the time. Don't use them...ever.
3. God, that one character is really annoying. Maybe I should kill him
Let's face it, you're gonna run into that one player whose character is just annoying. You know, the guy whose character has a 3 INT and must always do the absolute dumbest thing in EVERY situation? Or the guy who has acquired that one magical item that he uses as a crutch for perceived invincibility? There's nothing wrong with humbling these characters in games, to be certain, but resist the temptation to simply develop an encounter with the sole purpose of killing the character. It's hard to resist the temptation to simply have the character killed off so the sessions are less annoying, yes, but remember you're the referee and judge, and as such you should always remain fair and impartial. These types of characters will eventually be weeded out by other players if not by simply their own incompetence. I recall years ago, one of our friends was running an extended campaign, and one of our players ran a super annoying red-haired female thief named Mitra. She was always doing things which got us in a jam as a party, and really never showed any true benefit to the group. She was always conspiring behind the party's back, etc. So, what did we do as a group? We sold her character into slavery the first chance we got. The DM didn't have to worry about taking the character out. We did it for him as players. Yes, it resulted in some tense moments between the players, but shit happens. The point is, you as the DM should remain fair, even when one of the player's character is an annoying pissant.
So, these are 3 pitfalls I think are important to avoid in GMing. What are some of your own? Write up your own entries, and comment with the link to your blog here, and I'll compile them in one big post. Again, I'm interested in hearing your own takes on common pitfalls to avoid.