The Pond has been host to a male Wood Duck this winter, who must believe he's a Mallard, since he's claimed a female Mallard as his mate. (Children's story anyone?)
Cross breeding between Mallards and other ducks, while not common does occur more often than you might expect. The 10,000 Birds blog has a nice piece on the subject in 2007. We'll see later this spring if this strange pairing leads to hybrids.