That is, the standard syntax forms must be implemented as if they used hygienic macros; it is not actually required that they use them. For example, (let ((eqv? #f)) (case ...)) should not throw an error of the form "#f is not a procedure". A few Schemes, notably Bigloo, have this problem today.
Neither R5RS nor R6RS requires this, but I think the Principle of Least Astonishment does.