When I started working on a three-storyline draft for this blasted novel, I realized the need to map it out, and lo and behold! The mapping process led to a massive solution to a problem I couldn't figure out and a far more interesting ending to one of the storylines, all in one! Two birds, one stone, nice and tidy.
And how people start writing without having some idea of the ending blows my mind. Tried that twice, and both projects fizzled out for lack of direction and purpose. Getting to write the ending is one of the most effective driving forces for writing all the rest.