Did I understand it right that the inability to locate a single object for a single customer affected all your customers?
Almost right. The Tarsnap server aggregates together data from multiple machines into a single S3 object; I don't know how many users had data stored in that object, but it's probably more than 1 and less than 10.
But the problem wasn't really caused by the object going missing; rather, it was caused by S3 doing Something Which S3 Should Never Do, plus the Tarsnap server code not being designed with that possibility in mind. I've since adjusted the code so that an error like this will be handled less severely.
(That said, I doubt I'll ever see this S3 glitch again -- I got a phone call from Amazon providing some additional details about what caused this and it was clear that they were taking it very seriously.)