Kenneth G. Wilson (1923). The Columbia Guide to Standard American English. 1993.
prophecy (n.), prophesy (v.)
Although in the past the noun and the verb have been spelled both ways, today they seem quite settled, with -cy for the noun, -sy for the verb. Their different pronunciations seem not to confuse: the noun is PRAH-fuh-see, the verb PRAH-fuh-SEI. The problem is only for the poor speller.