In later Latin, /b/ between vowels merged with /w/, eventually leading to forms like modern Italian avere from Latin habēre. This only happened within a word: illa bucca became Italian la bocca, not *la vocca.
When these sorts of sound changes are in progress, though, they tend to ignore boundaries. For example, in modern Castilian Spanish, /b/ becomes [v] between vowels, and they will indeed say [b]oca but la [v]oca. It's usually only once the actual phonemes change that word boundaries are respected: in Judeo-Spanish, this change became phonemicized and thus no longer applies in cases like la boka.
So, do we ever see any evidence of B and V being confused word-initially, when the previous word ends in a vowel? For example, do we ever see la via spelled with a B or la bucca spelled with a V, before this change had become fully phonemic?