Who hasn't had one of these? They are among the most maddening of patients, because they are absolutely convinced that something organic is going on with their skin, and often their PCPs have also bought in to their story.
Multiple ulcerations and scars in an odd distribution (why do these lesions stop in such a sharply demarcated fashion?) give it away. When on the back, look for lesions in places they can't reach. It is rare to be evenly distributed across the chest in women, and although common on the face, it is pretty unusual around the eyes.
Any patient who tells you they see bugs or worms coming out of their skin is de facto factitial, as is anyone who brings in specimens of bugs they get out of their skin (unless, of course, they are real bugs, such as crab lice, which earn them a pass in my book. I'm just sayin', show me the bug or beat it.)
I really do feel sorry for these patients, because they are being driven mad by these perceived infestations, but they really should be another specialty's problem, not yours.