The steepest part can be invisible from the place where the plan was made.

Plans get made where the ground looks flat. Distance smooths a route, so the map, the spreadsheet or the conversation over coffee shows a manageable slope where the real climb is, and the hardest section only becomes visible once you stand at its base. That is less poor planning than a limit of the viewpoint. Difficulty is foreshortened from far away.

When you plan something, ask somebody who has done it where the hard part sits, and build in more time than the flat view suggests. Meeting an unexpected slope is normal rather than proof you misjudged everything.