((This is specifically written for fanfic, but it can work with pretty much everything.))
The following things do not exist: limits to the amout of physical damage one can take and survive; limits to the amount of emotional damage one can take without a breakdown; continuity; cause and effect.
"Canon" is only there to be screwed with.
Inventing new phobias = no problem.
Inventing past traumas = fine.
Inventing new painful, debilitating and medically improbable diseases = cool.
There can never be too much blood.
If the laws of physics get in your way, forget about them.
Death is a revolving door for villains, a brick wall for allies.
What would be considered "crimes against humanity" if done to real people, are "interesting plot devices" in fanfiction.
The Golden Rule of Torturing Characters: "Do unto your characters as you would have others do unto your ex."
Got more? Tell me.