Suppose someone creates an unauthorized music video to accompany a popular song, thus making an infringing derivative (assuming fair dealing or other exceptions don't apply in this case). This derivative author then gives you permission to copy their derivative. Independently, you have a legal right to copy the original song (an example would be you live in Canada where personal-use copying of music is generally legal).
It then appears that you aren't infringing the derivative author's rights since you have their permission, nor are you infringing the original author's rights since there's an exemption within the law.
It seems weird to me that there might be a legal possibility for you to own an unauthorized derivative by performing what appears to be a completely legal copy action, so I'm wondering if I missed something here.
Is the copying of the derivative work legal? Does the answer change depending on whether you have knowledge that the derivative is infringing? I'm most interested in answers covering Canada or the European Union.
Edit: I've since stumbled on a journal article by William J. Braithwaite that basically discusses my question in pgs 213-220, minus the portion about copying the original being legal. However, it discusses Canadian copyright law as it was in 1982 and is rather inconclusive.