Ever since the Sierra 10.12.2 update, the App Store app had been behaving itself. Gone were the previous failures to obtain updates, and it looked like this version of the app, 2.2, had fixed the problems which had been plaguing earlier versions.
Then today, there were three updates to be downloaded and installed. It took exception to one, and told me that an error had occurred, and I should re-install the app from my Purchases page.
Eventually, I coaxed it to complete what it says is the update. Only it then stuck, telling me that there was still one update to be downloaded, but that there were no updates available. It sat like that until the next updates were ready to be downloaded. Only then did it cancel that one remaining update.
So it’s going back on the bug list. If an app cannot count to three accurately, that’s a bug to me.