For GM's: NEVER underestimate the ability of players to derail your carefully crafted and detailed plots. A roleplay is fundamentally different from a video-game because even the greatest sandbox series just cannot provide the same level of freedom that a roleplay can. Never assume players will choose option A over B when they in fact decided to go for option T: [b]T[/b]he completely unexpected.

[img]http://theunshaven.rooms.cwal.net/RPG%20Motivational%20Posters/Player%20Characters.jpg[/img]

Don't try to negate the actions of a PC, even indirectly. Either by rapidly back-pedalling or introducing some plot device out of thin air just to preserve the scenario you had in mind. Player creativity should be encouraged, not strangled. Write a novel if you want a strictly controlled flow of the narrative.

For dice systems: The 'fail forward' system. Should for whatever reason the players critically fail at a task, do not make this a 'oh look how you suck' moment. When a master acrobat character rolls a one to jump on a table do not make him/her look like an incompetent fool and cause him/her to misstep like a child. Rather let him/her fail due to the environment: The table's leg snaps because it's rotten, or one of the baddies gives the table a shove at the critical moment causing the acrobat to lose his/her balance. Suddenly it isn't the player being an idiot it is the player simply being unlucky (reflecting the nature of a dice roll) or even better, the enemies being badass (makes it all the more satisfying to beat them).