In this weeks edition of Coin World
(Jan.28,2013) on page 66 is the "Collectors' Clearinghouse: Errors & Varieties" by Mike Diamond on Clamshell Errors.
Today I learned that my 2001 State Quarter I received in change is NOT a real clamshell error, but a Separation Error due to the outer clad layer not bonding to the pure copper core.
Also posted a 1908o Quarter that is a Clamshell Error.
[paper is placed inside the split on both coins]