I created some pages and added them as conditional content into some MasteryPath modules.
Later, I noticed them cluttering up one of my assignment groups. That's right: Pages, not assignments, automatically placed in an assignment group. I deleted them.
The next time I edited one of the MasteryPath assignments, these pages had disappeared from the conditional items in the MasteryPath specifications. A warning message said that some missing "assignments" had been deleted.
These pages have also disappeared from my collection of pages, and it appears I'm going to have to spend a few hours re-creating them.