Product and version FileMaker Pro 16v1
OS and version Mac OS 10.12.5
It's possible to create a style for button bar segments with no name. Which you then can't see or delete.
How to replicate
Create a new file.
Add a button bar to the layout.
Select the button bar and save the style as a new Style with a name, e.g. "Custom".
With the bar selected, make a change to the Button Bar: Segment formatting.
Delete the "Custom" style.
Run the DDR.
There is then a segment style that still exists, that has no name. With no name, it doesn't appear in the main list and therefore cannot be seen or deleted, but still appears in the DDR.
Various permutations of saving the style and or the theme appear to delete these, but it's also possible to save the style, but not the theme and then have another issue where you're trying to add a new style with the same name, and it won't let you.
Obviously there's some carryover where it's deleting styles, but keeping them around and just marking them as deleted in the DDR. I don't know what the intention of this is, and it causes issues for users of both the style selection, and the DDR as an accurate reporting tool.