Was looking for a thread like this so thought I’d start one. Inspired by my purchase that arrived today. I ordered a Base Set Machamp (Cosmos Holo), but I could only find it PSA 8 (I wanted ungraded), but it was a similar price so I thought what the hell. Spot the error:
It’s ITALIAN 
Did the seller advertise it as such? If not you may as well ask for a refund.
Not in the description or title, only in the photo (which I didn’t look at close enough!) never mind, I’ll keep it for the future!!
Countless “mint” cards off Yahoo Japan with little dents on them that you cant see in photos.
At Comic Con a couple of years ago I was flipping through a binder and saw a 1st Edition Gyarados. The person was selling it for under $100 so I thought I hit the jackpot. I paid as fast as possible and got the heck out of there.
Took a pic and sent it off to a friend to show off my amazing steal.
“Nice German card”.
Oops. It then proceeded to get a PSA 6 grade. Not such a steal after all.
bought a psa10 shadowless magikarp thinking i was buying a 1st ed.
Bought a Plasma Gale Charizard PSA10 wich had a 1st Ed. Tittle and it was Unlimited. lol… i didnt check the picture at that time and well…learn from my mistake.
Bought a booster box off a online Canadian store. Had the letters “FR” next to the title and I didn’t know what that meant. Realized it stood for “French” when it arrived. I still feel stupid.
Not much with Pokemon since I pay way more attention to what I’m doing. With Yu-Gi-Oh I constantly buy cards that say secret rare or from a certain set only to later see the picture is of some other common variant of the card. I do it all the time when I’m mass buying singles.
Amid the rush of all the PWCC auctions one month, I bid on a PSA 9 Neo Rev unlimited Raikou Holo thinking it was 1st edition. Didn’t even realize until it came in the mail and sold it for a loss a few months later.
A year ago I bought a minty Lost Remover Legend Promo on Yahoo Auctions for $30 thinking I got a good deal only to realize it was just the set version that is worth like $0.50 after it came in the mail. I went back to check the listing image and indeed the seller used the set version but advertised as the Legend Promo.
I bought a small lot of Japanese vending cards for my collection, but found out they were the quick starter versions instead. They don’t have the rarity symbols like the actual vending cards have and I wasn’t going to add them to the binder for consistency sake.
When I first got back into the hobby, I purchased a “unplayed” 1st edition holo Snorlax for $33. When it came in the mail, it was BEAT! But I still loved it and have it to this day because it was the first card I ever purchased in my collecting journey.
Do “not purchasing” fails count? 
I have a lot of set cards I accidentally bought duplicates for my binder sets when I forget to cross things out in a spreadsheet. I do not need 3x PSA 8 1st Ed. Light Azumarill but I’ve got 'em lol.
I bought 6 cases of Evolutions when I could have bought 5 bitcoin
I put a max bid on a PWCC listing and went to bed. Realized the morning after that I’d added a zero too much and some jerk had been shill bidding to screw me over. Ended up having to pay about twice as much as the current market value 
I was trying to complete my mastersets and had to buy some rather expensive league promo’s.
This holo pattern is called ‘Crosshatch Holo’.
I saw on cardmarket (site like TCGplayer) someone had this card listed with the description ‘scratched reverse holo’. I believed the seller did mean the league promo version, especially since the price was about the same.
Of course, I literally received a scratched reverse holo version of the card.
Once bought a fairly priced 1st Ed Base Charizard which turned out being an over priced shadowless. The price fooled me into not paying close enough attention at all the pictures, title, and description lol. The price was 20,000.00 which you can still see on pokemonprice.com.
I have a probably psa 6 Condition ungraded cosmos foil Machamp just pay me for shipping and you can have it if you want.
Bought my first sealed booster packs from Yahoo Japan through buyee this month, they got rejected from US customs and sent back to buyee. No idea what’s wrong and buyee wants me to contact customs when I’m not wasting my time doing.
So basically I should have just taken a $50 bill and set fire to it. Very frustrated, never using buyee again.