The extremely rare 1818 8 over 7 Half Sovereign is sometimes mis-identified due to existence of a variety that appears as if it could be due to a defect on the right side of the second 8, making that side of the coin appear fatter. There is however no trace of the upper side of the 7 which would be clear if this was indeed the rarity. This article illustrates the two types.
The George III 1818 8 over 7 Half Sovereign – An often mis-identified rarity. Graham Warboys.
BNS Research Blog 
Really useful article, thank you.
Excellent article, clears up all the questions of what the real variety looks like as well as the one experts mis identify.