OK, it means I almost certainly made an error in weighing out my grain. I just hope the error was in the base Pils malt, and not some astronomical error in the specialty malts. The beer is a Dubbel, and I knew before the boil that I was way over target gravity. I ended up diluting with an extra gallon pre-boil, and increasing my hops accordingly. Still, my target was 5 gallons at 1.069, and now I have 6 gallons at 1.073. I generally get pretty high efficiency, 85% or higher, but every now and again I get a batch near 100%, but never this high before. I just wish there were a way I could recheck my grain amounts. But I'm not complaining about extra beer!
(And just for the record, I am aware that it is possible to legitimately get greater than 100% efficiency. The highest I've ever heard of was the near 130% claim by DeKonick, in Antwerp. They grind their grain to powder, and use a high pressure filter to lauter. That's also why they have to filter the beer so much before it goes out the door.)