Some malevolent creature has cursed the cloth Giles uses to clean his glasses.  It could:

• Give him some particular insight into what he sees through them
• Do the famous rose-colored thing
• Decieve him - make something important less visible, or invisible
• The effects could get stronger the more often he cleans them

(For instance:  Willow says something about the cloth rather than Giles' being blind during Something Blue, and every time Giles cleans his glasses he becomes notably more inclined to support [Buffy & Spike's] true love.  Ends up driving them to church and officiating, blissed out of his mind.)