Blog of Exceptional Boredom

In Yoshi's Island, the core concept of the game's plot is that the Yoshis are transporting Baby Mario using a relay system. When passing through a goal gate at the end of any non-boss level, the current Yoshi passes Baby Mario to the next Yoshi, who runs in from off-screen just in time to catch Baby Mario as he is thrown.

Interestingly, despite the catching Yoshi not actually being playable in that level (since he would be the playable character of the next level), he is also coded to be a player character internally instead of an NPC as would be expected, which means that he is able to collect items by touching them.

While normally, there are never any items for him to collect, it is possible to spawn stars in such a way that they hop across the screen and touch the catching Yoshi, as shown in the footage. This then actually causes the stars to count for the level's totals. Note how the footage starts with the counter reading 28 while the Pink Yoshi is running towards the goal, while the results screen reads 30/30 due to the two stars the Light Blue Yoshi collected during the cutscene.