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When you forecast your own project, you think forwards through the plan, one step and then the next. The plan has no room for the meeting that slips, the supplier who goes quiet, or the people who keep working the way they always have. So the estimate comes out as the time the plan needs, which is rarely the time the project takes.
The odd part is that you already know this. Most leaders can name projects that ran late, and that knowledge sits right beside the new forecast without changing it, because this project feels different. The team is new, the plan is clearer, and the need is more urgent.
In that sense the planning fallacy is base rate neglect turned on your own plans. The record of how long similar projects took is available, and the plan in front of you crowds it out.
It's strongest when the deadline comes first. A target set by what the business needs, and a plan written to meet that target, will always agree with each other.
The practical tell is a timeline nobody has compared with the last project like it. If the only evidence for the date is the plan itself, nobody has tested it yet.
A new-business target the leadership team planned to hit in one year took us far longer. Accounts were falling even as revenue grew, so the board set £1 million of growth from new customers for the year ahead. It backed the target with a sizable incentive, expecting the extra earnings to turn us from farmers of existing accounts into hunters of new ones. The whole plan rested on that one assumption, and it was wrong: the sales team kept farming, and getting the incentives right took two years on its own. By the time we reached the £1 million mark, four years had passed and I had risen to the position of sales manager.