You can't sell them or pass them off as real cards, and you can't register them in sanctioned tournaments
But if you special summon Red-Eyes Black Dragon, it comes onto the field with double its ATK

Madison Li 1 Liberty Prime, Recharged 1 Automated Assembly Line 1 Brotherhood Scribe 1 Overencumbered 1 The Prydwen, Steel Flagship 1 Sentry Bot 1 Dweller's Journey 1 Curie, Emergent Intelligence 1 James, Wandering Dad 1 Nick Valentine, Private Eye 1 Synth Infiltrator 1 Assaultron Dominator 1 The Motherlode, Excavator 1 Plasma Caster 1 Synth Eradicator 1 Arcade Gannon 1 Electrosiphon 1 Red Death, Shipwrecker 1 Rex, Cyber-Hound 1 Sentinel Sarah Lyons 1 Shaun, Father of Synths 1 Sadistic Simulation 1 Brotherhood Vertibird 1 T-45 Power Armor 1 Ferrous Lake 1 HELIOS One 1 Austere Command 1 Open the Vaults 1 Mechanized Production 1 One with the Machine 1 Wake the Past 1 Mystic Forge 1 Panharmonicon 1 Solemn Simulacrum 1 Steel Overseer 1 Clifftop Retreat 1 Exotic Orchard 1 Glacial Fortress 1 Irrigated Farmland 1 Prairie Stream 1 Skycloud Expanse 1 Spire of Industry 1 Sulfur Falls 1 Temple of Enlightenment 1 Temple of Epiphany 1 Treasure Vault 1 Paladin Danse, Steel Maverick 1 Nerd Rage 1 Robobrain War Mind 1 Bottle-Cap Blast 1 Elder Owyn Lyons 1 Behemoth of Vault 0 1 C.A.M.P

Marrow-Gnawer is the most popular rat commander and inarguably the most stylish
Look at hand effects let you rubberneck, some cards let you cheat out permanents without paying their mana cost, and infinite hand size cards let you keep more resources than normal
These are specific cards that are available now, though these are also types of errors you could see on other cards