Breather Boss: Everyone at Junk City is considerably easier than the preceding boss, Garurumon. It's possible that they're even easier than the opponents in Beginner City.
Complacent Gaming Syndrome: Most players picked Patamon for the second partner, on a far higher frequency than every other partner selection events, thanks to storyline flow, popularity, and subjective gameplay quality.
Complete Monster: The Player Character, heavily implied to be Hiro again, comes back to compete in a card tournament that takes a sinister twist when some of the villains from Adventure return. The last boss is apparently VenomMyotismon, but he is abruptly hijacked when Analogman returns as a ghost and possesses the vampire. VenomMyotismon's mind is killed, but his body is now used by Analogman as a suit. Analogman challenges Hiro again, and loses, but before he can act on his revenge plan, he is sealed away in the "Deep Net." Analogman returns again in Digimon World -next 0rder-.
Game Breaker: Download Digivolve, Rosemon's Lure Card, Hacking and the 7-S cards in general.
Some support effects borders on this, especially those from Rare. The most prominent example however, is Toy-Agumon's support, by discarding one random card from your hand, you reduce the HP of both sides into 200. Yes, including Ultimates. This turns the Big Bad that uses The Computer Is a Cheating Bastard to the max into a total joke.
Yellow Banana deserves a mention too. For half of your HP, you counter any attacks. In practice, it mean that if it's not countered, you will survive the next turn on top of possibly dealing damage to the opponent.
One of Gatomon armor digivolve, Tylomon have a X attack which have the special effect of doing triple damage to red opponents. What make it game breaking is that unlike other partners, you don't have to equip a part that will lowers your X attack to have such an effect on Tylomon. Add a part that boost your X attack and the damage against red Digimon will skyrocket. Just watch out for counterattacks.
Good Bad Bugs: According to its description, the Special Digivolve card allows you to digivolve your Digimon into one of a different specialty, adding 20 DP to the amount normally required for digivolution. What it actually does, however, is reduce the required DP by 20. Additionally, Special Digivolving a Champion which was not digivolved from a Rookie (making its stats halved) will result in an Ultimate with its proper stats.
Junk Rare: The Fake Sevens and the 12 Digi-Jewels cards are only obtainable via fusion mutation, an event that only has a 1% of chance of happening while fusing cards. Trying to get all of them requires a lot of time, patience and spare cards from the player and their effects are so low better results could be achieved by using some of the most common cards in the game. Fake Sevens is easily the worst of them, as it does absolutely nothing.
Nightmare Fuel: The ending gets really dark. After defeating VenomMyotismon he starts repeating the same lines over and over. Then the game starts being hacked by a man known as A which causes the screen to go black and later all the colors change. At this part of the game, the player regains control but can neither leave the town nor save progress making this reminiscent of Survival Horror.
Let's not forget A's VenomMyotismon avatar which looks creepier than ever.