In the middle stages one may fail to anticipate a step, mistake how close a wall is to lean against, or misjudge the position of a chair when trying to sit down. They may even see a chair and initiate the process of sitting long before they reach the chair. Dad's earlier falls were mostly due to misjudgment and mistaking other objects for chairs. Later he would see the chair he wanted to sit in and begin to sit down. Unfortunately that meant the chair was still in front of him.
Misinterpretation is also a common problem.
For example, one might mistake something for a chair, when in fact it is not. I watched one lady try to sit on a cardboard box. Fortunately she was caught in time. Recently I was in a daycare center and suggested to one of the residents that we remove the magazine that he had wedged in the door to keep it open. He told me that he worked there and that it was necessary.