A quick google for me only came up with this, indicates hybrids may not really happen (they do in some birds) -
Hello all, saw two birds yesterday at Keldy, North Yorkshire and at first and second glance I decided they were an unusual leusistic Carrion Crow and didn't have time to observe them for long before they flew off. I've previously seen "brown" Crows and know there are all sorts of variations in...
www.birdforum.net
Did it have the long tail of a Magpie? Could be an abberrant Magpie, possibly some feather or mite damage to the head, or possibly (depending on how sure you were it was magpie-like), an escaped cage-bird of some kind roughly resembling a magpie. If you see it again a pic would probably help, even if from a mobile ...
And welcome to BF ;-)