Upon closing a Microsoft Word document, I frequently get the following message:

Microsoft Visual C++ Runtime Library
Runtime Error!
Program: C:\PROGRAM FILES\MICROSOFT OFFICE\OFFICE\WINWORD.EXE
abnormal program termination

Microsoft Word then closes down and I have to restart it to continue working.

I always save a Word document before closing it, so I have not lost data due
to this problem, but the problem is so pervasive and annoying that I would
simply like to know:

1. What is an "abnormal program termination"?

2. What causes "Runtime Errors"?

3. What can be done to stop them?

Thank you.