I am trying to determine the principled limits, if any, on when a person may be held financially liable for damage to another ("the victim"). I'm specifically interested in the case when damage is not reasonably foreseeable, and the person's mental state is good (that is, he is not trying to perform any bad act). As I understand it, if the victim has a strange and rare medical condition where he bleeds if touched even slightly, the person is still liable for damage inflicted on the victim, even if the consequences of brushing up against him could not have been known by the person.
There are various defenses against liability, such as contribution on the victim's part to damage, or lack of factual causation. I assume that the victim did not contribute to his problem, and an act (including omission) of the person did cause the damage. Is it ever a defense to argue "it was just an accident"? I'm looking for general legal principles, not the willingness of a particular judge or jury to overlook causation of damage in a particular case. Relevant citations would be especially useful.
To point to additional examples of the type that I have in mind, assume I have a tree on my property, which looks entirely healthy. There is undetectable insect damage to the inside of the tree which weakened it, and in a freak wind storm (winds of 60 mph suddenly arose, in an area that is not windy and prone to severe wind storms), the tree was blown down -- striking the neighbor's house, causing damage. Am I liable?