Have you ever been asked if you're a seat-of-the-pants or outline writer? I'm an SOP all the way. Outlining unwritten chapters is the most boring thing in the world to me. I had to do it in college, and I've never done it since. However, for writers who may shy from outlining, like I do, but you have a scene in your head you’re afraid you may forget, there's a solution that's not outlining: write out of order.
When I was in the process of writing Into the Darkness, I knew the beginning and the end, and I wrote it in exactly that order. Then, I filled in the events between the beginning and the end. Along the way, the deliciously creepy scene at the Thieves’ Guild popped into my head long before I'd even gotten chapter 10, so I wrote that down. After that, a bit of the showdown with Harbinger floated through my brain while making dinner one night, so I dropped what I was making and jotted that down.
If you have a scene in your head that is just too perfect to not scribble down somewhere, do it. Drop what you’re doing and write that scene. Plug it into the main story later.
Having a scene that comes further down the road may actually help you fill in the rest of the blanks more quickly. Just remember where you saved the file or put the piece of paper on which you jotted down the scene.
Sometimes, the creative process doesn’t happen in a neat and linear way. Don’t try to force it to be tidy. You might even find the chaos to be liberating.