Yu-Gi-Oh! 10 Powerful Cards So Good They Were Banned!

Firewall Dragon, the ace card of the Vrains anime, is a Link 4 which can can target monsters on the field and or graveyard and return them to the hand for every card 'linked' to it. Then, if a monster it points to is destroyed or sent to the graveyard, it can special summon 1 monster from the hand. Although it's a pretty powerful effect, it became the perpetrator of many infinite loop effects with other cards, it could extra link, blocking off your opponents extra monster zones, but it was also a horrific extender and OTK enabler.

Due to its ability to recycle by returning cards to the hand and enabling summons, it allowed players to easily extra link by filling all the extra monster zones that, before the master rule revisions, meant that players were locked out of using their extra monster zones. Coupled with cards like Cannon Soldier it allowed the player to deal direct damage or 'burn' their opponent infinitely since it could special summon whenever a card it pointed to was destroyed.

Since you were destroying the cards yourself, you could create an infinite loop of destroying cards, burning your opponent and then recycling them to summon again, forcing Konami to ban it.