Learning errors is hard, but the key is knowing the minting process and being able to match what you think is an error to something that happens there. There's an endless number of ways a coin can be damaged that we'll never be able to identify. That's why you have to look at it as 'How could this happen at the mint?'
There are some great articles on the website to explain it. Look at the sticky at the top of this forum.
http://goccf.com/f/126