In most cases when a writer types "The End" on the last page of a manuscript, it's only the first draft of many. I have heard of writers who only do one draft of a book, but I've never actually met anyone who's able to do it. Not saying it doesn't happen, just that for most writers writing a book takes multiple drafts to get it to the point where the author feels it's publishable.
When I first started writing I would often get mixed up as to what draft I was working on. One time I somehow managed to get pages from a previous version mixed up with the current version I was working on. It was a mess. That made me decide I needed a better way to keep track of what number draft I was working on.
I started putting in the header of each page the title and "draft #" whatever version it was. While that worked most of the time, I would sometimes forget to update the header and end up with more than one version with the same draft number.
I decided there had to be a better way to tell by looking at the manuscript pages if I was working on the 2nd or 4th draft of a manuscript. Around that time, I came across an article / posting from Susan Elizabeth Phillips. (For those of you who aren't familiar with her work, Ms. Phillips is a best selling author who writes amazing contemporary romance novels. I highly recommend her books, especially Nobody's Baby But Mine which is one of my favorite all time books.) She mentioned that she prints out each draft of her manuscripts on different colored paper.
What a great idea. There'd be no way of mixing pages from one draft with another. Now I print the first draft on regular white paper, the second draft on yellow paper, the third on pink, and usually the final and fourth draft on green. I always use pastel colored paper. If you try to use the fluorescent pink, yellow, and so forth, the brightness will quickly give you a headache.
So if you also sometimes have an issue knowing what draft you're working on, try Ms. Phillips idea. It's worked for me. :o)