Planning fallacy
Etymology
Coined by Daniel Kahneman and Amos Tversky in 1979. The striking part is durability: people who have blown past every past deadline still lowball the next one, because they picture the specific project going smoothly rather than consulting how such projects usually go.
noun phrase · /PLAN-ing FAL-uh-see/
The tendency to underestimate how long a task will take, even when you know similar tasks have run over before.
What it names
Why the honest, well-meant estimate is almost always too optimistic — we plan for the best case and forget the friction that always shows up.
Example
The planning fallacy struck again — "a quick weekend refactor" ate the better part of a month.
Related idea
Optimism bias is the general belief that things will go well for you; the planning fallacy is that optimism aimed squarely at time and cost.