While editing a file(several) in SharePoint Designer I was trying to just check in the file. Nope. I jumped through several hoops and it would not check in or let me undo checkout. I was stuck.

Seems this is something of a legacy issue. The solution is simple if not a bit annoying.

1. Make a copy of the file.
2. You can now check out the original file. Do so.
3. Check original file back in.
4. To remain tidy, delete the copy you made in step 1.

Why does that work? The new copy of the file resets the GUID apparently and releases the death grip.