In the rush to "start the real work" or avoid the discomfort of including activities where there is a great deal of uncertainty, project teams often trivialize the impact of missed work on schedules, not to mention budgets, thinking they can always "fit it in" somewhere down the line with minimal effect. That's a dangerous illusion. There will always be activities that cannot be planned at the appropriate level of detail because they are dependent on outputs from earlier activities or because of other current unknowns. The answer is not to not include them or hope that you'll be able to accommodate them later but rather to insert placeholders based on the best available point-in-time information and an explicit statement that they will need to be refined at a later date (e.g., ends of phases).
It's critical to remember that a poor estimate might result in a 30% variance but a missed activity is will result in a 100% variance - a benchmark or reminder activity placed in the schedule based on the best information available at the time is better than nothing. Missing work or unrecognized dependency relationships or hand-offs) can have significant implications (e.g., the organization may not have the resources (people or budgetary) to complete the “found” work when it's ultimately revealed) and there may also be dependency implications from missing inputs resulting in still more schedule variances with possible threats to the project completion date.
What's the answer? Give up-front planning the priority it deserves. Strive for completeness, leverage lessons learned from prior similar projects on what was "forgotten" and use collaborative, cross-functional walk-throughs and scope/schedule reviews to ensure that all major deliverables and the inputs/activities needed to complete them have been accounted for in your schedules, budgets and plans.
https://www.projectconnections.com/articles/121807-pritchard.html - Using an inventory of stakeholders to ensure that all project scope and requirements have been accounted for.
https://blog.projectconnections.com/project_practitioners/2013/09/a-fresh-look-at-the-5-whys.html - Try these 5 why-like questions to improve your scoping, requirements-gathering and stakeholder identifying activities.
https://blog.projectconnections.com/project_practitioners/2009/07/mind-the-gap.html - Using gap analysis to help your team avoid falling into “gaps” that could cause your project to fail (like forgetting about necessary data).
https://www.projectconnections.com/templates/detail/product-definition-sucess-factors.html - A checklist of critical success factors for defining a product, and a worksheet for assessing the current state of your project's information.
https://www.projectconnections.com/templates/detail/deliverables-definition-form.html - An outline for defining the contents of "deliverables" your team needs to create during a project, and information about how a well-defined deliverable can make your projects go more smoothly!