This is not a phrase people want to hear about their coin collection, but unfortunately, this happens every day to collectors who don't know what to do, but mostly, don't know what not to do. Some of these mistakes, surprisingly, are also done by professional collectors and dealers.
Never Touch Your Coins
Just something as simple as handling a coin can be costly. Of course, if you coin roll hunt and collect circulated wheat cents, for example, you are going to do no real harm by touching the coins. However, if you collect rare coins and mint state or proof coins, just touching the coin can damage it…and damaged coins cannot be repaired. Your hands contain oil and dirt, even if you just washed them. That oil and dirt will then touch the surface of the coin, causing small hairline scratches and may cause discoloration and unattractive toning over time. When handling your coins, always hold them by the edges. There is a debate as to whether or not gloves should be used when handling coins. Some collectors believe that you should always wear gloves, while others suggest that you should handle them by the edges with clean hands. That way, it is not as easy to accidentally drop your coin, which is generally much worse than touching them. Also, cotton gloves will cause small hairline scratches, which is not something you want. Personally, I wear cotton gloves and only handle the coins from the sides while holding them above a soft cloth to prevent damage if accidentally dropped.
Never Clean Your Coins
If you're into uncleaned ancient coins, this does not apply, but for most collectors, you should never clean your coins. Cleaning coins will not only decrease value, but also cause many fine scratches and strip away the original mint luster. Some people choose to use strong acids or even ketchup, both of which are bad ideas. If you think it makes the coin look better, it actually does, but only to the untrained eye. All experienced collectors will be able to see immediately that the coin has been cleaned. The scratches are a giveaway and the luster is another dead giveaway. Mint luster is generally seen in a “cartwheel” effect. Cleaning coins will alter this and the light will not reflect off the coin properly. Even many beginning collectors will notice that something is not quite right. If sent to a professional grading service, cleaned coins will not be graded and will be given either “Details” or “Genuine” status, only verifying its authenticity. There are some “coin cleaners,” but I would never recommend using them.
Never “Doctor” Your Coins
Coin doctoring is a form of altering a coin to make it appear more valuable. That could be by adding a mintmark, removing a mintmark, changing the date, making the coin look mint state when it's not, etc. Coin doctoring is a deep field that I am happy to get more in-depth with, if my readers would like that. Essentially, though, it is altering the coin in a way to make it “appear” better. When buying coins, you do have to be aware of coin doctoring, because many are even able to get past the best coin grading services. Some of it has to do with removing spots, changing toning, altering the surface, and many even add putty to a coin to make it look different. Many coin doctors are really bad, but unfortunately, a lot are pretty good. If you see any claims about this or people saying they will make your coins “better,” run away.