The file is already being used in another verification.

When adding a second verification using only a meta tag, the save failed saying "The file is already being used in another verification." (referring to the verification file field).

In case that isn't clear, here's what I did:
1) installed 6.x-1.0-beta3 (fresh installation, not upgrade)
2) "Add verification" -- Bing, entering only a meta tag (successful save)
3) "Add verification" -- Google, entering only a meta tag (failed)