The most likely reasons are if it's sacrificed, if it's legendary and another legendary permanent with the same name is controlled by the same player, if it's a planeswalker and another planeswalker with the same subtype is controlled by the same player, if it's a creature with toughness 0 or less, or if it's an Aura that's either unattached or attached to something illegal
The backsidea 4/3 trample that should mostly be indestructible during your turnmuch like the front side, is fine, although not game-ending or anything like that, especially in a format where everyone is playing Sheoldred, the Apocalypse
This means that you dont need another card to win the game once youve got infinite mana
This means that these cards are legal for use in any tournaments where the original printings are still legal.
Reserved List and early Magic history
The rest is put at the bottom of the library in random order