I often write things out of order (in both senses of the word).
This can happen for two different reasons:
1. I want to get something right, i.e. I want to be sure that it will truly work, before writing the narrative that leads up to it.
2. I tend to write parallel threads. (Okay, being honest here, I write (it's more than a tendency)
too many parallel threads.) Some can be discarded as being diversionary (
), but others are required by the story. For this reason. I've had to chop the text into sections so that now my original first chapter (rather, its distant descendant,) is now to be found nearly half-way into the book.