Time Machine Failing to Index File (Error 11)

This is not a question. This is information in case it might help someone else.

I suspect this occurred because I restarted my computer while it was doing a backup.

Anyhow Time Machine backups were failing with error 11. Repairing the Time Machine external device didn't fix the problem. Reading the logs I noticed that a file was failing to index in some folder on the TM drive which had "inprogress" in the folder name. Using finder I navigated down to that folder on the TM drive and it was a file not a folder. So deleted it. And emptied trash.

Tried to back up again and it still failed but now referenced a different file. Both files referenced (the one from the previous error and this one) belonged to a folder extracted from a zip file. So deleted that folder on the original drive and emptied trash.

Time Machine is now working.