There are generally two roles within an accident scenario: one party who was more careless, and one party who was less careless. According to the carelessness rule of personal injury liability, the party who was more careless should pay reparations to the party who was less careless in proportion to how much more careless they were. This applies to most situations, including when the injured party had also been careless, or partially contributed to the event of their injury.
Carelessness can be construed through many forms. If a property owner has not properly maintained his building or grounds, then he is liable for any injury that may befall guests. If a manufacturer or seller delivers a defective product, then they both may be liable for any injury caused by the defect. If an employer fails to prevent an employee from being careless and causing injury to someone else, then the employer can possibly also be held responsible.
If more than two parties are involved in an accident, then most states mandate that any party involved pay full reparations to an injured party. This is likely to require some negotiation between the parties and their insurance providers in order to figure out who should pay whom.
Careless parties may not be held responsible for injuries caused by dangerous property if the injured party had no business being there; e.g., trespassing or snooping.