In LR 7.0.1, re-reading metadata from a photo can create a zombie keyword that can't be deleted from the Keyword List. This behavior does not happen in LR CC 2015.12.
To reproduce:
1. Create a new catalog.
2. Import a photo that previously had the keyword A assigned to it and saved to its metadata.
3. Observe in the Keywording and Keyword List panels that A is assigned to the photo.
4. With the photo selected, select A in the Keyword List panel and click the "-" button to delete it. Observe that the keyword disappears, as expected.
5. Do Metadata > Read Metadata From File, and observe that A comes back to the Keywording and Keyword List panels again, as expected.
6. Select A in the Keyword List panel and click the "-" button to delete it a second time. Observe that A does not get deleted from the Keyword List.
I haven't found any way to get rid of these zombie keywords -- restarting LR, assigning them to new photos, exporting as a new catalog, deleting preferences, etc. don't work.
Here's a video demonstrating the problem: https://www.dropbox.com/s/p5w6djvbjxc4mg9/delete-keyword-bug.2017.11.04.mov?dl=0
Tested on LR 7.0.1 and LR CC 2015.16 / MacOS 10.12.6.