There isn't just 'one cause' for every problem. In this case it works
backwards, the solution to your problem can help determine the
underlying cause to why you encountered the error in the first place.
For example in the case of Normal.dot corruption, I firmly believe the
underlying problem is due to third party global template/add-ins that
modify your Normal.dot each time Word starts.
I personally do not use add-ins that modify my Normal.dot. Such
add-ins can easily lead to a bloated template which can ultimately
lead to the corruption of your Normal.dot. A corrupt Normal.dot is not
"normal". I am a heavy user of Word, my Normal.dot is fairly
customized, and I would honestly have to sit down and think of the
number of times I have encountered a corrupt Normal.dot. Off-hand I
would say maybe twice in all of my years of using Word. I know one was
due to a macro virus but that's all I can easily recall.
To protect yourself you should have "Prompt to save Normal template"
turned on under Tools/Options/Save. If you did not modify your
Normal.dot, such as customizing a toolbar, creating an AutoText entry,
etc. but you are prompted to save changes to Normal.dot upon exiting
Word then you should investigate the reason behind the modification.
If it is an add-in causing the prompt then do not use the add-in. A
properly designed add-in should not modify your Normal.dot for reasons
previously noted.
Now if that wasn't what you found to be the solution, let us know what
you encountered and we'll go from there.