For example, Obosh, the Prey Piercer can only be used as the companion of decks that exclusively feature spells with odd-numbered mana costs, and Yorion, Sky Nomad can only be used as the companion for decks featuring 20 more cards than the minimum for their format
Ground Seal turns off Snapcaster Mage, Deathrite Shaman, Unburial Rites, Seance, and part of Angel of Serenity
Speaking of Consider, Councils Deliberation is also great if you can get it into the graveyard without paying its casting cost
Once Wizards started a second print run in North America, the base set was broken up into Shadowless (for the first run) and Unlimited
Using a microscope or magnifier, you may find out that the original card has a multi-layered print