The frustrating truth is that rare valuable error coins are just that: rare. Most oddities seen on coins are post-mint damage or deliberate altering. Here are some of the most common kinds.
True doubled die coins have certain characteristics that a practiced eye can spot, but some damage to coins can look like doubling to those that haven’t seen it before. This is especially true of dings or dents near fine details, like Lincoln’s ear on the penny, which can look like faint doubling.
This type of error does happen at the Mint, but is not the more highly prized double-die effect, when a coin is struck from a die on which part or all of the image has been engraved twice. Machine doubling occurs when the die or coin slips during the strike, producing a faintly doubled image on the coin.
Found a coin with an unusually thick rim? It’s probably been through a clothes dryer or is the product of someone trying to make a coin ring. Here’s a great post with more info on this kind of damaged coin.
Usually caused by grease in the die during striking (though it can also be caused if the die is chipped); this a very common error, and does not increase the value of a coin. (These coins do make for nice conversation pieces, though!) The Crazy Penny Guy blog has a great post about a very strong example of this kind of coin.
A coin with a face on both sides must be a significant error, right? Not usually. In virtually all cases, these are altered coins, often used by stage magicians. One good giveaway is if the two faces have different dates.
Fakes and Counterfeits
Found an old coin with really clear details? Check to make sure it’s a real coin; the market for older coins has become flooded with many fakes and counterfeits. Coin Week has an excellent ongoing series dedicated to identifying counterfeit coins.
The best way to make sure you’re getting a real coin and not an altered or fake coin is to buy from a reputable dealer.