Halitosis.

For someone for whom social awkwardness is already a problem, a lifelong problem (or, more particularly -- I don't know if this is always the cause of social awkwardness, but it is in my case -- a neurotic fear of having no control over others' impressions of me) bad breath is the worst nightmare, the ultimate test. It's like walking through life with shit smeared on your face, maybe, probably, shit which is only perceptible to other people so you don't know how often it's there or how much, and there's no way for you ever to know.