The Sunk Cost Fallacy is a real pain.
It's a cost that you sustained in the past (be it time, money, energy, whatever) and cannot be reversed, cancelled or undone.
The fallacy is that yet knowing about the sunk cost of a project, you are often tempted to justify your future investments because of your past (most likely lost) investments.
The sunk cost should not influence your future decisions, yet it does. And the outcomes are often delusion, sadness and stress.