Dissociative Identity Disorder (DID) is pretty much the most fascinating DSM diagnosis of all. Formerly called Multiple Personality Disorder, DID is a rare disorder diagnosed when a client presents more than one discrete identity or personality state that recurrently takes control of his or her behaviour. Each identity has a distinct and enduring way of behaving, perceiving, and interacting. Correspondingly, in addition to significant memory lapses and time unaccounted for, symptoms of DID include things like being told that one behaved extremely uncharacteristically, not responding to one's name, and being frequently accused of lying.
DID is associated with early traumatic experiences, particularly childhood physical or sexual abuse, often by a parent or other trusted caregiver. The hypothesis is that dissociation is an extreme response to severe trauma: the mind splits off the memory and awareness of the abuse; the memories go into the subconscious and eventually emerge in another personality, meanwhile allowing the original identity to exist as though untraumatized.
Without forgetting the anguish and suffering inherent to DID, I can't help but be impressed by the brain's capacity to protect individuals from their own terrible experiences by creating a separate personality to whom the terrible experiences happened. Some research has even found evidence for differences between identities in handwriting, and in physiological variables like heart rate and blood pressure!
Clinical psychology doesn't get more amazing than that.