Having switched to a capped gas supplier six months back, I was surprised to get a 'Notice of High Bill' letter from them - especially since I have given them 'real' meter readings every time.
It turned out on investigation that their meter reader had been round and read the initial '0' as a '1' - adding 10,000 cubic thingies to the total.
Based on how many we actually used, I estimate that would have been an extra £30,000 for this quarter. I'm quite glad they have a sanity check built in to their system!