However, it appears to have happened at least
once before. And AnimeSuki re-listed it.
Apparently, if a company smells, on second thought, that a series will not be that big of a deal, could drop the license. At that point, a fansub group - say, the one previously subbing that series before the licensing - could re-pick the shows and continue the work on it. Clearly, it would seem quite rare and peculiar as an event. And as for legal and illegal, the substance doesn't change from how it was to begin with.
(sorry for Akira, got a bit carried away by Cats' post
)