1972 Lincoln cents:
Below are comparison shots of notable regions that differ between the two known master dies, which in turn led to the working dies, that struck Lincoln cents in 1972 (including proofs). At least 50% or more of all struck 1972s have the type of doubling observed below. Often they are sold as doubled dies on eBay. Be careful, these coins carry no premium at all.
1972 Lincolns with master die doubling are often, and I cannot say why, thought to potentially be 1972 die 004 or 005. Scrutinize potential finds by comparing known die markers, images of MDO and images from known sources such as coppercoins.com before posting it in a new thread.
The easiest differences lie in the bulbious heads of the three Ts (from LIBERTY and TRUST), followed by the funny topped B in LIBERTY, and then distinctive doubling on the upper portions of IN GOD. Die state, hub state and circulation wear can mask the doubling, making identification harder.