Photoshop file "could not save..." "...because write access was not granted"

I've been seeing this error a lot lately:

Could not save [file] because write access was not granted

I've checked all the standard reasons why permission to save a file might be denied:

• The file is not open in any other application


• The user does have access to the drive and directory


• No other users on a network are accessing the file


• The file is not read-only


• No other save operations are happening


• The connection to the drive is fine

I've seen this happen on Lion and Mavericks. I've only ever seen it on a Mac, and only ever with Photoshop, with .PSD files.