I had deleted the files in trying to reload the workspace, even though I had shelved the changes. Then VS2010 thought those files were still pending to save. I didn’t need that, so I had to figure out to “undo” the changes in Pending Changes.
Then I could unshelve.
It thought I had two ops (unshelve, commit-for-add) going simultaneously, and I thought I had only one op (unshelve).