When I started working in architecture, architects were often classified as project managers. The more technical were called technical project managers and the the more business orientated were classified as Business Analyst/Project Manager. one element that has remained from that time is that an architect is expected to be good at project planning. The problem (and yes this is not an opportunity) with project planning is that it is less precise than whether forecasts 200 years ago. Typical examples of horrible planning include the Sydney Opera House, the Eurofighter, the new Berlin Airport, the now existing Denver Airport and many more examples. I am sure that you can add your story here as well.
Now usually many of us try to solve this problem without actually realising that the planning fallacy can have many reasons and a whole lot of people have so far unsuccessfully tried to solve this. Most techniques build on on or more root cases and especially five years ago everyone claimed to have finally solved it. However we still see cost and time overruns. So this why I will not try to offer a solution but instead discuss commonalities that may be responsible for the planning fallacy.
The first one is the explanation is that of reductionism where a complex task is dealt by putting it into smaller pieces, but then often forgetting the influences of system interdependencies. Sometimes it is possible to bring them into the planning, but only if it is well defined closed system, as to plan in certainty in an open system is a near impossibility.
Another explanation that is often mixed up with a solution is of impression management, where people regulate a goal and implementation by social interaction. The problem is just that the medium of language and other means of social interaction is understood different by each actor and as such makes a common planning near impossible.
Focalism proposes that people fall victim to the planning fallacy because they only focus on the future task and do not consider similar tasks of the past that took longer to complete than expected. This often happens if the planning is under a strong political influence to come up with a certain outcome or in answer to a given business case.
Throwing new people on an existing project does not have a single word to it but is often a key factor that destroys any planning as with new people especially in high quantity planning is often reversed. This is more true for a project that is not following a stand pattern. So throwing large resources on a housing project where all houses are build in the same way and where the new resource has already been building on another housing estate with the same design leads to a much smaller impact than putting vast new resources on a project with a unique design.
The planning against nature is another classical cause for the planning fallacy as it ignores certain boundary laws. The classical anecdote for this is to plan for birth by letting 9 mothers do it in one month instead of one mother in nine month. The sad thing however is that a lot of parallelisation in the planning is just following this pattern.
Empowerment is another interesting cause for the planning fallacy as it is often also the means to overcome planning fallacy. The dilemma is that if you empower people a certain task can be speeded up, but it is likely that someone else who was also empowered does the same. Additional with empowered staff it is very hard to control costs. However not empowering people will usually means that very little is done and a lot of people will wait for decisions doing very little.
Finally (in my list) linear scaling is one of the key causes I have experienced as a cause for the planning fallacy. However you may argue that this is just a subset of the reductionism cause. Usually nothing scales linear as early examples of the empire state building or the golden gate bridge have shown.