Sonic the Hedgehog once ran so fast that his appearance changed forever. Sonic was attempting to prevent a time beam from destroying his home world and unraveling time itself, transforming into three temporary elemental forms in the process, even before his look changed forever. The Blue Blur didn't reach this incredible speed on his own, however. He had a Super Chaos Emerald strapped to his back the entire time. Fans first reading this story in Archie Comics' discontinued series were undoubtedly baffled from the start, because writer Karl Bollers actually wrote issue #71 backwards - an attempt to emulate the effects of the time beam.
The time beam itself wasn't enough to destroy an entire planet - that was a danger only because of how the beam would interact with another, already active time warp. Right before Robotnik died at the end of the 'EndGame' arc, he blasted the Freedom Fighters' hideout, called Knothole Village, with his so-called Annihilator. Rather than living up to its name, the Annihilator shunted the village three hours into the future. The interaction of the new time beam with the futurized village threatened a chain reaction that would have destroyed the entire planet and unraveled time itself. So, naturally, Sonic needed to ensure that didn't happen.
Even though Sonic is the fastest thing alive, he wasn't fast enough to achieve this on his own. So he strapped a Super Chaos Emerald to his back that - when combined with him attaining Super Sonic, then the even more powerful Ultra Sonic forms - allowed the hedgehog to reach insanely high velocities. Incredibly, this combination also led to Sonic's Ultra form manifesting in different ways based on where he was running. When zooming through the Great Desert, he assumed the form of the red-hot Solar Sonic. He later became Tundra Sonic while skating through the Southern Tundra. And, finally, the lush trees of the Great Rainforest caused him to transform into green Eco Sonic.
It didn't take long before Sonic the Hedgehog had reached such high speeds that time actually began to accelerate, a phenomenon that caused the time beam to divert away from Knothole Village and head towards Sonic. The moment the beam hit the Blue Blur, Knothole Village was brought back into the present, and Sonic emerged with a new design, losing his black pupils and simple footwear for green eyes and buckled sneakers. The change itself is treated as something of a mystery, though with the strong implication that like the elemental forms, this new look signifies Sonic has adapted to the incredible speeds his time-lost mission made necessary.
Really, the story served as a way of updating Sonic's look to be more in line with his appearance in the videogame Sonic Adventure, hence why it was here to stay. Despite that, it was a cool detail that Sonic 'earned' his new look through an act of heroism, even though his tantalizing elemental forms and the powers he was implied to gain with them never appeared again. Sonic the Hedgehog's epic, large-scale adventures were what made the Archie Comics run so special, and after saving Knothole Village - not to mention the world - the changes made to the Blue Blur were a fitting tribute to how far he'd come.