It is mortifying if a novelist describes a character's eyes as blue and later refers to them as grey. In order to avoid such mistakes I fill in a detailed character profile, which includes appearance, for main characters and a simpler one for minor characters, which also includes their appearance.
In my new historical novel Monday's Child there are many minor characters. Instead of depending on my memory, I have a card index in which I record the names of those who have a very minor role.
These aids to memory are very useful. I remember reading a historical novel which began with the main character, who was called - say - Lady Violet. Half way through the novel Lady Rose was introduced. What had happened? The author had changed her mind about calling the main protagonist Lady Violet and re-named her. Unfortunately, she forgot to change the name in the first half of the novel and - amazingly - neither the contents editor nor the line editor notice so the name Lady Rose replaced Lady Violet when the novel was published. Of course, this is an extreme example, but novelist's beware, but I wonder what the readers made of it.