All plans will deviate from the plan.
This has been my experience even with plans prepared by expert project managers.
Moreover, long-term plans are more than likely to change drastically as the world around us changes.
Unforeseen events are 100% likely and 100% likely to impact even the most detailed plan.
The pandemic was a great example of that and major events are more common than we realize, even if it does not affect the entire planet all at once.
The most common mistake I have come across with planning has been the estimated time to complete i.e., the due date.
Almost every first reaction tends to be based on the desired state, what seems or sounds reasonable and/or based on gut instinct.
These unrealistic plans lead to what I call planning pains.
In these instances when I have asked for an objective justification or explanation, I often encounter silence.
At times, I have even counted the hours certain tasks or steps could take and mathematically have determined most plans underestimate the time required by 3X.
Not to mention other factors such as people getting sick and/or going on leave or even being pulled into other projects.
For these reasons, when I do project planning, I usually only start with the first three steps and factor at least 2 standard deviations from the calculated expected time.
Granted, this technique is applied to projects whereby I can actually provide an objective basis for a mathematical calculation for the duration of a step or task.
From this, you can see that I am not a fan of arbitrary expected due dates and believe planning is in fact imperfect. Yet, I do find it useful to highlight and identify the high-level steps of any project.
I always call them high-level to start as I know other more specific and detailed elements will emerge and have to be factored in to the plan as we progress.
Essentially, I start with the Macro and add the Micro as we move along the planning horizon.
I was once asked to complete a major project in 2 weeks. My preliminary calculations estimated the best-case scenario would take 3 months to complete.
Needless to say, when I relayed this to the project manager, they were not happy.
Given this project was not date driven and more results driven, my final answer to the project manager was, “it will take as long as it takes.”
In case you are curious, it ended up taking 6 months, the project achieved what it was meant to, all risks were avoided, and no harm was done.
If and when possible, avoid planning pains by not making unrealistic plans.