Originally posted by: Y The Alien
"'shiryoku' does not at all refer to power of observation - just means eyesight in Japanese, and has nothing to do with powers of observation."

I don't know what the word for metaphorical word would be, and I copied this from somewhere else so I'm relying on their Japanese translating abilities. Happy?
I'm not sure about the japanese translations so I can't really tell who's right. If Y is right that would mean that the Sharingan has yet another weakness, in addition to sucking up the user's chakra. But if Y is wrong and that Itachi did overlook something about Kakashi, it would make way for a more interesting story due to Kakashi's new jutsu/sharingan secret.