Thanks for the advice, but opening up the Normal just to modify it is not what I'm looking for here and is not really compatible with my workflow. I rarely run into a situation where I have to edit the Normal per se. When a situation arises during the editing session of a regular document that requires a new customization (e.g., a new button on a toolbar), that's when I want the change to be reflected in the Normal.

I think your answer is probably right. I installed a new add-in a few months ago, and it somehow coincides with the last modified date of the Normal, making it a strong suspect. (I won't name names here though.) Like you said, it must have been badly designed, because I had been using other add-ins without a problem before.