Hopefully this will be less of a problem going forward with ebooks but the internet is full of stories of writers with several profitable books published who ended up sitting on novels for years when the horror market collapsed, good well edited books that went on to be profitable when "Urban Fantasy" started selling.

Or just a case of publishers not wanting to publish more than X books a year from a given writer, regardless of output.

Those are books of course. Movie scripts are notorious for floating around Hollywood for years, sometimes decades.