On the covers of the first two Discworld books, Josh Kirby actually drew Twoflower with four physical eyes. Consensus on alt.fan.pratchett has it that Terry was trying to get across the fact that Twoflower was wearing glasses (‘four-eyes’ being a common insult thrown at bespectacled folks), but that Josh Kirby simply triggered on the literal text and went off in a direction of his own. Whether this action essentially shows Kirby’s interpretative genius (the KirbyFan explanation) or his inability to get the joke / read very carefully (the NonKirbyFan explanation) is a matter still under discussion.