This is a largely frivolous entry.
The logo used for this site is a Penny Black with the security letters FO for forgery. There is, of course, no such thing as the letters ranged from AA (top left) to TL (bottom right). Thus, there was an OF, but no FO. A more convincing forgery would have used a nice OF example and reversed the letters, thus ensuring consistency of colouring and size. As no OF example was found on the internet the day the job was done, a stamp with a pleasing cancellation was chosen, the F swapped and the O taken from another stamp.