Essentially a face-to-face phenomenon, this is an extra unit slipped onto the board, or exchanged for a piece of a different type (say, an Army for a Fleet) or colour. If it goes undetected, the player controlling it will often endeavour to embroil it in the development of the game so thoroughly that, even if spotted, it becomes difficult to rectify the mistake. House rules may vary in such circumstances: under some house rules, it is possible to swap F(Tus) for A(Tus), order it to Venice, then "spot" the mistake and have it changed back to a fleet. In others, the unit will be moved back to Tuscany and made a fleet once more, or have to remain an army for the rest of the game. Where there is not a GM, this sort of duplicity is thoroughly within the spirit of the game, for all that the more "sober" games-player may disapprove.
However, in postal play, this sort of thing is only possible if the GM makes a mistake or is misled by a player (perish the thought!), and since it is generally acknowledged that deception of the GM is unacceptable, the latter is likely to result in the player being ejected from the game, the zine, and perhaps worse.