Seen it happen. It is easy to fix though. Decline the license agreement and it reverts to previous OS in about 10 minutes, and nothing is damaged.
Unfortunately, even if the older version is successfully restored, the user can still be left with a painful hole in the pocketbook. The Windows 10 upgrade can take up to 10 gigs of the users data allowance. The millions of users out there working under capped data allowances could see an expensive surcharge from their carrier even if they decline the agreement and have their older version restored.
This happened to my boss in the middle of the work day. Shut down his machine for over 90 min. and had him really, really cheesed off.
I have a feeling the office will be going Mac shortly.
But there is a possibility of damage. If you have a nearly full hard drive the win10 installion crashes. Then you are stuck with hundreds of unmovable hidden files and a hard drive too full to run correctly. Had it happen to me and I had to buy a new backup drive to clone the filled drive and then sort out the mess.
It tried on my computer, but I believe I was able to shut the download process down before it was completed