The ERROR node is a constant node that always evaluates to the internal error flag value. It is typically used to simulate an error in a calculated data link during knowledge base development.

The ERROR node always returns the internal error flag value. This value is used internally to indicate networks or sections of networks that are not valid. When a node receives an ERROR value from one of its child-nodes it also will pass the ERROR value unless the path of the child-node is inconsequential, such as an unused path in a switch, if, or SOR node.