This is an interesting thread. My ex-DP displayed some of the behaviour that other posters have mentioned - false claims of bereavement, abuse, minor and pointless exaggerations to major complex and plausible stories that fooled many people for years, including medics, psychiatrists and other professionals - she even went so far as to call me from abroad and claim her waters had broken, her labour had begun and she was about to give birth to our daugther, even making the sound of painful contractions at regular intervals - our daughter was actually born a month later.
Other posters have noted that lying is associated with Personality Disorder. This is true to an extent - however, long and complex lies that seem to serve to portray the teller as a persecuted victim or stoical hero in the face of relentless bad luck and adversity, are more commonly characteristic of Factitious Disorder - this includes long lasting, complex lies based on a matrix of truth and that have a self-aggrandizing quality that define the condition known as Pseudologia Fantastica.
When the person tells lies for material gain (eg Social Security Benefits, drugs or to avoid military service) this is more accurately described as Malingering Disorder.
Munchausen's Syndrome is a specific sub-set of Factitious Disorder in which the person adopts the 'sick role' in order to receive love and attention by faking or inducing the symptoms of an illness (which may be a totaly fake illness they don't have, or may be a real physical illness the symptoms of which they can simulate or aggravate - eg a diabetic with co-morbid Munchausen's may deliberately misuse their insulin).
All these disorders are serious mental illnesses and difficult to both detect (even by medics) and treat - particularly because in many cases, when confronted with their deceptions, the sufferer will simply flee and carry on their factitious way of life in another location with new friends.
The lies in these disorders are proactive (eg not simply told to avoid a row or confrontation) wilful and conscious and often very complex. They are not delusional or subconscious.
There is some research to suggest a link or co-morbidity between Factitious Disorder and some Personality Disorders (notably Borderline Personality Disorder and Anti-Social Personality Disorder) - though this is certainly NOT to say that people with PDs (which are very common, affecting a couple of million people in the UK) are more prone to lying that anyone else.
See this page for more info.