Short description of error
some curves get wrong normals, which makes grabing with 'normal' as orientation wrong.
Exact steps for others to reproduce the error
Open attached file
see how the translate manipulator is aligned to world axis instead of normal axis.
Note that this curve was created by the dxf importer, I don't know if it's related. Recreating the curve from scratch gives correct normals, but it's very time consuming.
Recalculating normals doesn't help.