dark knight yugioh card man from nowhere Blackjack Rants: Reviewing Yu-Gi-Oh Monsters #5 This is of Secret Rare rarity Yu-Gi-Oh! Dark Knight Gaia card

Think the infamous Black Lotus

Memnarch or Tiamat, Energy Tap turns into quite a strong ritual in a color thats not really known for playing this type of effect

Bulbapedia has an extensive list of corrected and uncorrected errors going all the way back to Base set including information about possibly misidentified errors such as the Base set Red Cheeks Pikachu which is a less common version of the Yellow Cheeks Pikachu

Some of these are just very good early game cards that allow you to fix your draws, while others have very splashy effects like giving you extra turns or allowing you to steal some of your opponents cards

Blurry, off-centre or oddly coloured backs (too purple, too green) are dead giveaways