"But as strange as this was, this was not what frightened me. What frightened me was that they were absolutely thin. It was like they were as flat as a set of clothes that had been ironed. Their faces and bodies were entirely flat - no contours. The nose did not jut out, nothing."
Generally, I take those "let me tell you what happened to me" stories with a grain of salt. They're often vague and can in no way be confirmed. And while that may be true for the chilling account shared by Lon over at Phantoms and Monsters, the tale of MIB's and mysterious, thin humanoids is at least interesting and fairly well-written. In a note following the submitter's tale, Lon divulges an interesting bit of corroborative evidence.