While testing overwriting imported obj files, I stumbled across this:
On the left, a pipe created in Rhino; on the right, that same pipe exported as NURBS obj and imported again.
RH6 file: Vertical Pipe.3dm (101.0 KB)
Results in RH5:
Importing that RH6 OBJ file into RH5 results in the blue surfaces in the middle, whereas roundtripping the green pipe in RH5 (export to obj and import again, all in RH5), results in the orange pipe on the right: