This one might be my favourite piece of rules weirdness yet: I make Spellweaver Volute into an Equipment and end up casting a creature that is already sitting on the battlefield.
The setup
Spellweaver Volute, from Future Sight, enchants an instant in a graveyard and copies that card whenever you cast a sorcery. I start it on an Opt, then transform what it is attached to:
- Liquimetal Coating makes the Volute an artifact.
- Saheeli, Sublime Artificer turns it into a Servo, so it falls off the Opt.
- Swift Reconfiguration makes it a Vehicle that loses its other card types but keeps its abilities.
- Bludgeon Brawl then makes it an Equipment, which I attach to a Platinum Angel.
The payoff
Now I cast a Ponder. The Volute triggers, and thanks to rule 303.4m an ability that refers to the "enchanted object" refers to whatever the permanent is attached to, even when it is no longer an Aura. So it copies the Platinum Angel it is equipping, and I get to cast a creature that is already on the battlefield. I do not know of anything else in Magic that lets you do that.
There is even a rules argument that, per 704.5e, the copy should stick around even if you choose not to cast it. Rules lawyers are welcome to fight about that one in the comments.