A new level of dumb.

Taxpayers to pay for tax glitch.
Unbelievable. I imagine that the tax booklets were sent in an envelope anyways, however, at least I'd hope.
It's likely that they weren't, and were just presorted bulk mail that were addressed and sent in a catalog-fashion.
I worked at a mail house once, and know that the printer probably used a database program for bulk mail (probably the same one I did,) that has all the necessary addressing information to send to the receiver, including the social security number. Those were probably accidentally left on as information to print within the printed address.
Or at least I'd imagine, as a worst-case scenario. How else would personal information appear on the outside of the booklet?
Yay for screw-ups!