
I think incorrect usage of terminology is the problem here- you are not differentiating true errors from die varieties or normal wear and tear. Die chips, cracks, "missing" letters, etc are all part of the normal minting process as dies are used and abused. Die chips are commonly confused with extra metal but no extra metal is actually added to the coin, that is a very rare error indeed. A chip in a die will cause a raised blob on a coin, same as a die crack forming a raised line. Unless very large, these are not generally considered errors and have no premium regardless of what some unscrupulous sellers on
ebay may lead you to believe. "Missing" letters are caused by a die getting clogged with grease, metal shavings, or other types of dirt so the impression does not fully strike up. Unless a coin is missing major portions of detail, these are largely ignored.
The "extra" trees and islands on Minnesota and Oregon quarters are die varieties not errors, specifically doubled dies. Many people will call these coins "errors" but that is a misnomer and only confuses people. Die varieties are traceable to a particular die used to strike the coin and all coins struck with that die will have the feature. These coins tend to be very collectible. Over mintmarks and repunched mintmarks are examples of other die varieties.
True errors are also very collectible, especially with the
State Quarter series. Many errors on
State Quarters are worth much more than a similar error on a 1965-1998 quarter. This can include many thing that can occur in the minting process- incomplete(clipped) planchet, off center, broadstruck, cud(large die break emanating from the rim of the coin, it signals a die failure), split clad layer, etc. The coin that ron6788 posted is an off center strike and is a true error.
Numismatics can be a confusing hobby but it is also very terminology-dependent to accurately describe everything that can happen to a coin. Without a proper lexicon, we are nothing more than collectors of shiny metal discs
