A significant class of failures in interactions with data base query systems are attributable to misconceptions or incomplete knowledge regarding the domain of discourse on the part of the user. This paper describes several types of user failures, namely, intensional failures of presumptions. These failures are distinguished fran extensional failures of presumptions since they are dependent on the structure rather than the contents of the data base. A knowledge representation has been developed for the recognition of intensional failures that are due to the assumption of non-existent relationships between entities. Several other intensional failures which depend on more sophisticated knowledge representations are also discussed. Appropriate forms of corrective behavior are outlined which would enable the user to formulate queries directed to the solution of his/her particular task and compatible with the knowledge organization.