In buying and selling coins, we get to see a lot of mint errors--a ton of them in fact, from off-metals to off-centers, and everything in-between! This gives us a lot of insight into what error coins exist, their condition, and also lot of practice in determining authenticity and figuring out how any particular error coin was made. This is all made possible because we get to hold the coins in hand and closely examine them, and as said earlier, because we buy and sell a lot of errors (plus we see a lot of errors we don't buy at all.) So why do we bring this up? To make the point that the more coins you can see, the better off you will be in terms of building a high quality collection. Not being a dealer, this can be hard for many collectors to do, but there are ways to help get some good hands-on experience. Here are some ways you can get some "experience" without having to actually buy any coins.
When you get a chance to attend a major auction, take the time to review all the mint errors in the sale, even the ones you aren't interested in buying. Look at them under a loupe, and think about how the error was made. Look at the coin's surfaces, and notice the subtle differences in the error. Review the grading services' description in the coin; did they note all the errors on the coin, or did they run out of room on , and left other errors unmentioned (that happens a lot in fact.) Look at the grade that was assigned--why was the coin given the grade it was assigned? Errors are graded generally much less strictly than non-error coins, and so it's good to understand that so that you don't overpay (or underpay) for and error based on grade.
Also, major auctions allow collectors a chance to see major errors that they wouldn't normally get a chance to spend 10 minutes reviewing--so take advantage of this great opportunity to see some major errors.
Do you have a friend with an error collection? Take the time to get together with them, and take a good look at their coins. Look at the attribution, grade, authenticity, and surfaces of the coin. If you're one of the fortunate few who have access to one of the really major error collections our there, definetely spend all the time you can with reviewing those coins since some of those coins are doubtless unique or close to it.
Although this is not as good as in person viewing, online major auction sites often have excellent photos, and allow collectors the ability to see millions of dollars worth of error coins from their computer screens. Some sites include Heritage Auctions, Stacks/Bowers, Great Collections, eBay, and we have the Sullivan Numismatics archives as well.
Although dealers are often busy at coin shows, and may not have the time due to the busy nature of coin shows to allow you to physically look through their entire inventory (we can vouch for that!), you can simply through their display cases at the coins for sale, and get some good experience that way. Many dealers will have hundreds of thousands of dollars worth of inventory at coin shows, giving a unique opportunity to see some rare mint errors.
This may seem obvious, but buying coins for your collection affords an opportunity to build your knowledge. Some collectors quickly look a coin over, then throw it into a slab box in a bank vault, and never really spend much time looking their coin over. Don't be that collector, but instead, take the time after buying a coin to carefuly review it, consider all it's attributes, and then put it away into your safe deposit box (or wherever you store your collection.)
The more you know about error coins, the more your will appreciate your collection, and the better collection you will be able to build. Knowledge is key to building a good mint error collection, so learn all you can about errors, whenever you have the opportunity, by carefully considering all their attributes.