Wow, good to know. Thanks for the information! I'm working to help a team of many editors who edit in Word, and have to copy/paste from Word into a web page for their final reports. I managed to automate a lot of their endnote processing, renumbering, and handling duplicates. But when they pasted from Word to the website, we were getting problems with these hidden bookmarks.
I'll have to make sure none of them are using TOC in their documents. Or maybe I can find a way within my code that's causing these hidden bookmarks, so I am only deleting the ones created during my scripts.