10 Mistakes to Fuck Your Campaign

Not a long time ago, 'bout 2 weeks or so, I've participated in 2 game sessions, as a player. He opened the game well, stretching his great idea into a good game, and the session ended top notch. I was happy, 'cause I had a great game. He was happy, because he GMed well, very well actually, even for a not first time GM.

Then came the next session. I, and the other 2 players, came for the game with the expectation from last session, but for some reason or another, the game didn't stand for what it had promised. I started to think about it, trying to figure out how such a wonderful game can collapse in one session. My thinking brought me to these points:

1. If enough story advancement has been achieved, don't add new players. True, there are game where you can do it, but they are the exception and not the rule. Adding new players changes all of the game up to this point.
2. If you add a player, make sure the player's play style is not that different from the rest of the players. First sessions are there for finding this formula for what kind of a game it will be. Adding a player with a different play style makes the adjustments much heavier. In that game, 3 players who are more free-formy and less technical were joined by a very technical and analytical player.
3. If adding players, or if players are missing, think about a rational explanation for why they weren't there. I stumbled with this aspect a lot in my GMing "career", but saying that the character was in the bathroom for example is not that good of an explanation. I try to do something else: I pretend that the character spoke what s/he had to say, and if there's a battle, the character is taking to the side one of the enemies... The character is still there, but in the background.
4. The PCs are there to get the limelight, not the NPCs. In that second game session, there was an NPC on our side who was unstoppable and just a mind-blowing killing machine. We, as players, didn't know what to do about him as we try to stop the same villain he's trying to stop.
5. Players, you aren't there to do some photosynthesis, so don't be vegetables. In Israel, we have a term for the player whose there for saying the "I attack" when is being attacked and that's all, veg (although in Israel we call the player "a flowerpot", or aziz (עציץ)). Players should act, not stay back and wait for others to tell them what to do.
6. References are good, sheer copying is bad. Don't copy entire stories from other places without some adjustments. Better yet, don't steal characters and put them in mid game.
7. Consequences are not a dirty word. If the PCs do a bad thing, or use a power inappropriately, don't let them get away with it that easy, or they'll do it again.
8. On the other hand, don't forget to appreciate the good deeds, or they'll be less and less common.
9. Make sure the players know what to expect for when entering the game. If you want a gritty fantasy game and your players want a heroic one, one of you will have to compromise, and it will probably be you. Make the compromise beforehand and the game won't suffer for being this late.
10. Don't railroad the players, and most of all, don't think that only a particular way (or ways) will solve the story, it will just get into a halt. "No adventure survives first contact with players", so don't make it too dear for your heart.

And, that's it. How about you? What mistakes did kill your campaigns?