Continuity mistake: When Ashitaka spots San spitting a mouthful of blood out, the number of large teeth on San's necklace changes from four, to three, to four.
Continuity mistake: When Ashitaka is swimming to Lady Eboshi's fortress, he sees a dead man floating near him. He wasn't there in the previous shot.
Answer: You can see part of the ooze flowing across Moro's neck, thus it gets severed.