It's implied that George and Martha have been playing out the game of having a childin real time—the entire eighteen years their son's been "alive." Every day. Unable to stop. No wonder they drink.
And in the final sequence, Martha's on the verge of asking George if they can invent ANOTHER imaginary child. George quietly but firmly puts the kibosh on that idea...but who's to say they won't change their mind later?
The story of the car crash is told three different times throughout the play. In the first two tellings, George describes the crash as an accident, while Martha implies strongly that it was deliberate. The final time, everyone knows that the circumstances of the story are fictional, but George is using the story to deliberately "murder" their fictional son, meaning that this time, it wasn't an accident.
