Shifty Doppelganger

Cheating the cast comes at the price of keeping it: spend four mana to exile the Shapeshifter, drop a fat creature out of hand with haste for a single swing, then watch that creature die at the next end step while the Doppelganger crawls back to the battlefield to do it again. The mechanism is a rental, not a reanimation. You get the attack trigger, the haste alpha strike, the enters-the-battlefield payoff, and then the body leaves before your opponent untaps into it. That sacrifice clause is the whole transaction's pivot point: it lets the design hand you something far above three mana of impact without ever letting you keep the asset. Two corners of the rules text reward attention. The reentry is conditional on the sacrifice actually happening, so a creature that has gone elsewhere by end step leaves the Doppelganger stranded in exile. And the enters-and-leaves loop is built for cards whose value is front-loaded: an enter-the-battlefield trigger you want repeated, or a creature whose death trigger pays off when the built-in sacrifice fires rather than one asked to fight. As a way to deploy haste it predates the cleaner haste-cheats that came later, and its insistence on sacrificing the creature makes it read more like a recurring entry-and-exit valve than a way to durdle a giant onto the field. Built around the right triggers, it is an engine; cast naively as a way to play things early, it is a glorified one-turn rental with a clumsy refund.