Look at the "2 cents" black overprint on the stamp. You'll notice that the "c" and "e" and possibly the "n" are inverted. The "c" almost resembles "o" whereas the "e" is clearly inverted. I noticed the error recently while sorting out my North Borneo stamps collection. Due to its black(ish) background, I didn't realize this for so many years and could not recall where I got the stamp from. In fact, I almost sold this stamp together with the other cheap North Borneo stamps in ebay months ago. Nevertheless, as common in philatelic world, there is always a potential find even among junks and rubbish.

In Stanley Gibbons catalogue, a stamp bearing a normal overprint above is catalogued at £27 for mint and £15 for used (SG 186). There is no mention of this specific error even though there is a mention of the "s" being inverted and valued at £100 for mint and £95 for used (SG 186a). Inverted "s" may also means that there is a slight possibility for the other alphabets to be inverted as well. In Scott catalogue, inverted "s" variety is catalogued at $87.50 for both mint and used. I haven't seen any inverted "ce" before, nor have I seen any inverted "s". So I don't know for a fact how much the 'real' values for these verieties. Even though the stamp might be a real rarity, I haven't ruled out forgery although to the best of my knowledge, the overprint has never been forged. If you have some insight please let me know :)