I was just listening to The World at One. The main story was that Gordon Brown had made what he called an "unintended mistake". This gladdened me as it would have been odd if he had made an intended mistake. I'm not sure that a mistake can even theoretically be intended, unless it's a cartographer's folly.
My word! I was just googling for a link to explain what a cartographer's folly is and could find almost nothing. So here goes. Maps are very easy to plagiarise. Rather than going out with theodolyte and measuring rod you can just reprint the Ordinance Survey map and claim it as your own. If the OS try and sue, you just pretend that you really mapped the area and that your map is identical to theirs because it is of precisely the same place. To stop such skullduggery, in every map you see published there will always be one small, deliberate error. It will be so tiny that it could never harm an orienteer but it will be there like the slight assymetry in a Persian rug and it will be enough for the original cartographer to be able to identify his work and sue.
I have often wondered, spending as much time as I do sniffing dictionaries, whether there is such a thing as a lexicographer's folly.