Personally, I've never had coins switched on me (fingers crossed), but I've had coins show up out of the blue I never ordered! It turned out the original buyer had the same last name as me and the dealer mixed us up when labelling the package.
The problem is not that mistakes are made; we all make mistakes. The problem comes when a dealer expects the customer to bear the cost or trouble of correcting the dealer's mistake or decides that his time is too valuable to correspond. There is a VCoins dealer who forgot to send me the coin I bought with immediate Pay Pal on two consecutive orders. Both times he apologized and sent the coin after being reminded. This is a hobby for me. Getting attitude or getting ignored reduces the fun of my hobby. I do what I need to do to increase fun and that includes not repeatedly ordering coins that may or may not come. Call me unreasonable. I will point out that earlier this year a dealer I have dealt with for 28 years made an error for the first time I can recall in nearly 100 transactions. I was taught years ago that it takes a hundred Atta-boys to offset one Aw-shucks. That means he still has about one credit remaining.
I have had it happen a few times. The most recent time I bought one particular RR bronze and the seller sent me another one. Unfortunately he couldn't find the coin I actually ordered so we worked out a deal for a partial refund and I kept the coin I ended up receiving.
This actually just happened to me today! A few days ago, I ordered volumes IV and V of the Ancient Coin Collecting series. I saw that Wayne Sales (who wrote them) was selling autographed copies on Amazon, so I sprung a few extra bucks and ordered them directly from him. He mixed them up and sent me volumes V and VI instead. When I sent him an email, willing to return VI (which I already had), he told me to keep it and that he would put IV in the mail on Monday. A real class act! In other news, I've really been enjoying reading this series...
I bought approx 300 ancients this past year for my collection. I have not had any wrong ones. Only one was lost. I have probly purchased the same value in other classes of coins... none wrong. Most purchases online.
Really, I've only bought "36" humble coins this year (so far) ... Oh, and two brand new vehicles (*sigh*)