The outline helps, of course, but what I find slips through the cracks is the weather.
I still remember the first time I read through a novel with an eye to seeing how the author did something. It covered many years, and I wanted to see how it handled the changes in season. At the end, I observed -- it didn't, after all, and I had just not noticed it didn't include them.
I tend to. But at the moment, when the landscape is drab brown with crocuses and daffodils and some bright blue flowers I glimpsed out of the corner of my eye brightening up, that's what comes to mind when my hero and heroine are trudging down the road. It takes a minute before I remember that it's late spring, or even summer, and the heroine had been able to gather wildflowers and make them both flower crowns before they go into a city adorned with floral decorations. Daffodils and crocuses would not suffice. (Dandelions would. But having already seen the first dandelion of spring, I will not put the annoying yellow things in my story.)
Ah, well. This story's going to be fun because I've already decided to give it a climate unlike the one I live. Winter is the rainy season, and they don't get snow. . .gonna have to keep track of things like that.