1) It's not resolvable without a large project. You can't 'spotfix' the problem in the game without doing something like going into world builder and reducing the amount that's about to overflow.
2) Hard to say. Usually as soon as it breaks, the next save won't load because of some corrupt value in the save asking for an index that doesn't exist when it goes to load, finding a null reference.
3) Any tracked variable in the game suddenly becoming anywhere between -some 2 million to positive some 2 million. Unit strength, population on a city, the reference value to the type of terrain a unit gains double movement to. Take your pick of ANY variable and one overflow on any game variable and innevitably other variables will start to go completely random.