Just like how any game has broken cards. They slip under the radar due to being underplayed/changed at the last minute/designed before really knowing how to balance cards. To borrow from MTG, some great examples are
Tarmogoyf (Changed from 1GG to 1G at the last minute, making it incredibly undercosted and playable, especially in blue decks that played green for this one card),
Umezawa's Jitte (Which again, is undercosted for what it does and turns the game around unless your opponent can deal with it), and
Skullclamp (Due to a last second "nerf", going from +1/+1 to +1/-1, allowing you to kill any x/1 creature you controlled for 1 mana and draw two cards!).