Crash in MSHTML.dll

[IUnknown]

The MFC application that I have created hosts three instances of the web
browser control. It interacts with those controls using a number of
published interfaces: IWebBrowser, IHTMLDocumentX, IHTMLElementX, etc.
About one out of 20 times I exit the program, there is a crash in
MSHTML.dll (the memory at address XXX could not be "read".)

There is no strange behavior or broken functionality, just the crash at
shutdown. The crash does not seem related to user actions in the program I
created. I.e., I can't find any way to reproduce or debug this problem.

I'm at a loss. For now, I just call exit() so my application will not
crash, but I would like to fix this problem. What should I do?