For centuries, the Rig Veda had been passed on orally within a few families, with learners running it all together in a single flow, not even thinking of word boundaries, mixing up the sort of saṁdhis you would only get at word boundaries with saṁdhis that would happen within a word. When the Rig Veda was later compiled and edited into collections, each compiler-sage had to work out how to break up this flow into discrete words (pada-pāṭha).
Hence the emphasis on having a "guru" for study of vedas?
