Super Mario 64 · Nintendo 64 · 1996
A reward for 100% completion: Yoshi waits on the roof of Peach’s Castle with a message from the development team, 100 extra lives, and a super-powered triple jump.
Super Mario 64 launched 3D platforming, and its designers hid a personal thank-you for the players who mastered it. Collecting all 120 Power Stars unlocks a cannon on the castle grounds that fires Mario onto the otherwise unreachable roof, where Yoshi — Mario’s absent companion throughout the whole game — is standing alone. Yoshi greets Mario with a note signed by the developers, hands over a stockpile of 100 extra lives, and grants a dramatically higher triple jump before leaping off the roof to the world below. The message famously contains a typo, reading in a rushed voice that betrays a development team working down to the wire. It is one of the earliest and most heartfelt "completion reward" eggs, a template countless later games would copy.
Collect all 120 Power Stars. A cannon opens on the castle grounds (the one in front of the castle, near the moat); use it to blast Mario onto the roof, then walk up to Yoshi.