In an input validation error, the input received by a system is not properly checked, resulting in a vulnerability that can be exploited by sending a certain input sequence. There are two important types of input validation errors: buffer overflows (input received is longer than expected input length) and boundary condition error (where an input received causes the system to exceed an assumed boundary). A race condition occurs when there is a delay between the time when a system checks to see if an operation is allowed by the security model and the time when the system actually performs the operation. In an access validation error, the system is vulnerable because the access control mechanism is faulty. In an exceptional condition handling error, the system somehow becomes vulnerable due to an exceptional condition that has arisen.
Source: DUPUIS, Clement, Access Control Systems and Methodology CISSP Open Study Guide, version 1.0, march 2002 (page 105).