Errors and varieties are two separate things, noga. An error is a unique coin that was created by some mistake in the minting process - that could be a clipped planchet, a coin struck twice, an off-center strike, a planchet with a missing clad layer, etc. A variety is a coin struck by a die that is distinguishable from other/the typical dies used that year - examples include doubled dies, repunched mintmarks, wide AM and close AM pennies, reverse of '89 1988 pennies, etc. There are also coins that straddle the line between the two like die breaks/cuds and clashes, which I've generally seen referred to as "die state" coins because they are not caused by any mistake in the minting process, but are also not like varieties in that the die that struck them isn't necessarily distinguishable from any other prior to the damage. The 1976 D has a famous doubled die obverse on the word "LIBERTY." As for errors, most errors can potentially occur on most coins/dates assuming there's nothing about the coin that makes a particular error physically impossible. I recommend looking up what some of the main error types are though so you don't waste time thinking every mangled parking lot coin is an error.