A Bucket Error is when multiple different concepts or variables are incorrectly lumped together in one's mind as a single concept/variable, potentially leading to distortions of one's thinking. Bucket Errors are related to Fallacies of Compression.
The term, Bucket Error, was introduced in "Flinching away from truth” is often about *protecting* the epistemology where an example is given of a child who refuses to believe that they made a spelling mistake. The child is unwilling to believe this fact because they believe having made a spelling mistake is incompatible with becoming a writer which is their dream. "Becoming a writer" and "never making spelling mistakes" are lumped in the same bucket despite in fact being separate variables.
Bucket Errors are similar to the concepts of equivocation, identification in Buddhism, or fusion/defusion in modern psychotherapy. See: Fusion and Equivocation in Korzybski's General Semantics.