The most frequent mistake I see in Project Planning is what I call “premature Gantting” — going straight to creating a huge Gantt chart (often in MS Project).
Why is this a mistake? Because a huge Gantt chart, with many lines of tasks, all precisely allocated to specific dates looks very authoritative. It gives the impression that you’ve spent hours working out the exact dependencies, estimating the length of tasks and precisely scheduling all the work that needs to be done and who needs to do it.
If you truly have done all that work, then perhaps you’re fine. But the reality is that most of us have ACTUALLY spent half an hour scribbling on the back of an envelope, or at best on a flip-chart, to get some rough stages and some key dates. All the detail was then made up when the Gantt chart was created, primarily because the software demanded it.
What’s the solution? Let the format of your plan reflect the stage of development that it is at. Internally, I will frequently go into a meeting and DRAW the plan stages on the whiteboard. For communication with the customers/stakeholders, I’ll create a simple plan (sometimes even a flowchart) that reflects the reality of the situation.
So far, everyone involved has appreciated the honesty — and it’s prevented projects going wrong at a very early stage in the game.