There's also the problem of material in the public domain being copyrighted. I recently heard Paul Jones expressing surprise that Mike Bloomfield's version of Blind Lemon Jefferson's "See That My Grave is Kept Clean" was attributed to A P Carter. I have heard that Carter copyrighted everything (having made minor textual changes) when the Carter Family first started their recording career. Dylan and Lightnin' Hopkins kept the Jefferson attribution.

A couple of years back, Ludlow Music won a civil case about a line in a pop song derived from a line in a Lowden Wainright, which was in turn derived from a Woody Guthrie song, both of whom are published by Ludlow. Unfortunately, the line in question was from a Guthrie cover of another Jefferson song, and in this case Woody had followed Jefferson almost verbatim.

At least (on my copy at least) the Dubliners attributed Child #274 as trad arr Dubliners.