The one that amazes me is the traditions where people learn the violin with no teacher. In former times, this was the norm in Shetland and Transylvania that I know of, and presumably other places in between: you'd just watch, practice at home (or up a remote hill in a barn) and join in when you could. In Shetland, this led to a distinctive style of bowing, since there weren't many players and once you got the basic idea you'd be playing along with them for life. In Transylvania (much larger place with musicians much more mobile) it led to utter anarchy with many different styles coexisting.