The World Health Organisation’s ICD-10 manual of diseases and health problems has a diagnosis of ‘Strange and Inexplicable Behaviour’ that gives, rather appropriately, no further explanation, except that it’s classified with code R46.2
It is from Chapter XVIII of the ICD-10 which tackles ‘Symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified’.
It turns out that the whole of Section R46 is a bit of a gold mine:
R46.0 Very low level of personal hygiene
R46.1 Bizarre personal appearance
R46.2 Strange and inexplicable behaviour
R46.3 Overactivity
R46.4 Slowness and poor responsiveness.
R46.5 Suspiciousness and marked evasiveness
R46.6 Undue concern and preoccupation with stressful events
R46.7 Verbosity and circumstantial detail obscuring reason for contact
R46.8 Other symptoms and signs involving appearance and behaviour
Many thanks to my friend and colleague Jorge who pointed out this little known and under-appreciated diagnostic gem.
Link to ICD-10 chapter with section R46.
Mind Hacks 
I think it’s time for the British to bring back the autopsy finding of “death by misadventure.”
Clearly these aren’t really diagnoses, but are presenting features that sometimes appear separately from the syndromes to which they could otherwise be ascribed.
They also seem to basically be describing Michael Jackson.
Which reminds me there was for a while in the southern US states, wasn’t there, a “diagnosis” of “runaway slave mania”.
All homosexuality was pathologized in the same way prior to 1973. The fact that many sexual minority members had no other symptoms to speak of beside same-sex attraction was beside the point.
http://drvitelli.typepad.com/providentia/2010/06/unmkaing-disease.html
They’ve got me bang to rights on half these – I’ll go quietly.
I vividly remember a diagnosis of “Death due to collission between airborne vehicle and tram”. That could on the other hand be a case of a yet to be explained memory perversion.