You could be looking out for mated pairs, brockages, capped dies, or counterbrockage. Other things to look out for in error world coins are indents, wrong metal/wrong planchet errors, bonded coins and spectacular errors. Other error specifications include mules, die adjustment strike, proof errors and transitional errors. Double denomination errors are difficult to spot. You could be looking out for struck fragments, fold-over strikes, errors on proof planchets and dual type. It is always easier to start collecting at a local level simply because access to global coins is difficult for most people. Trading at a local level is simple because each day you get access to a lot of loose change and this is not true for global coins. All world error coin deals need to be struck through off shore traders, reputed dealers, or collectors and could prove expensive to start with.