It's a stamp from someone, somewhere, for some reason, after it left the BEP. Not an error note but a definite spender.
Small shops in my neighborhood (especially foreign owned) mark the bills they receive this way. I understand it is a way they mark the bills to know that they checked the bill for authenticity when they first received it. I suppose that if someone claimed to have received a bill that was counterfeit, they would be able to know whether or not the bill ever was in their register.