Same thing
Hah, exactly the same thing happened to be last year. I hadn't put in an actual reading all year, so they were going on my estimates, and when I did put in a reading it was not only less than their estimated reading for that quarter, but less than the reading for the quarter before (owing to a new boiler I guess. And me wearing jumpers in winter). I was expecting to owe about £20; the actual amount I owed was -£12; my bill was something in the region of 10 grand. After looking at it for a few seconds and realising it couldn't possibly be correct, I saw the funny side.