The Special Digivolve card in Digimon Card Battle is supposed to allow you to digivolve your current card into a Digimon of a different specialty, adding 20 DP to the amount required for digivolution in exchange. That's how it's supposed to work. However, using the Special Digivolve card actually reduces the amount of DP required by 20. In addition, using Special Digivolve on a Champion stage card which was not digivolved from a Rookie (resulting in its stats being halved) made the resulting Ultimate Digimon have its proper stats.
In Yu-Gi-Oh! GX: Spirit Caller, the pack glitch. Apparently, whoever was responsible for the code for buying packs en masse did a horrible job of it; there's a fairly easy way to buy all or all but one of the packs in a set for the price of a single pack. This can potentially mean that you're buying 51 packs for the price of one.
In Yu-Gi-Oh! GX: Duel Academy, it was possible to give the Copycat monster an obscene amount of Attack points. At least one AI duelist (in this case, Blair) uses Scapegoat, which creates a group of tokens on the field. Bringing out Copycat and having it copy one of those tokens would, strangely, give it more than 30,000 points when it came time to attack. Note, however, that the exact circumstances that lead to the Attack increase are not known, so use with caution.
Burst Stream of Destruction should only be playable if you have a Blue Eyes White Dragon on the field, but the game doesn't check for it, so you got a unrestricted card (i.e. you can have 3 of them) that can destroy all monsters on the enemy field and is also fairly easy to get.
