I know tons of these for Sonic 1.
In this part of Marble Zone, you can actually break every one of those grey blocks (plus some additional ones below) with a single jump. If you do, you gain an immediate 10,000 points and a free 1-up. Nice!
Also:
- In both the first and third Special Stages, you can get to the area where the Chaos Emerald is without touching the controller at all. Just don’t press anything and you’ll end up next to the emerald, though you’ll need to maneuver Sonic to it manually once you’re there.
- Labyrinth Zone was originally supposed to be the second level after Green Hill Zone, but they changed it because it was too difficult. This is why both levels have totem poles and statues — LZ is supposed to be “below” GHZ.
- There are two versions of Sonic 1 with slight changes. In the original U.S. release, Sonic dies after being hit by a spike, then falling into another spike. In the later Japanese version, Sonic has invincibility frames between spikes, so he doesn’t die immediately if he touches two in a row. This was an incredibly contentious point in the classic Sonic fandom during the 2000s, and you can usually tell the quality of a SEGA collection by which version they use (typically the JP one).
- You have ten chances to get all 6 Chaos Emeralds — 2 for each zone — but you can’t access Special Stages after Star Light Zone Act 2.
- Sonic was originally meant to be a lighter shade of blue, as seen in the original Dreams Come True promo material, but he was made darker at the very last second because you couldn’t see him against GHZ’s sky. In Sonic Adventure DX, the 8-bit version of Sonic 1 uses the original colours.
- Plugging Sonic 1 into an S3&K cartridge gives you the Blue Sphere level select screen, letting you choose between a cool 16^10 levels at your leisure (as opposed to the pre-programmed single level that every non-Sonic 2/3 Genesis game gave you). You can’t play as Knuckles because his sprite effs up the game’s colour palette (though one of the Sonic Mania developers did prove this could be done through crafty ROM hacking).
- Sonic 1 and Phantasy Star 4 use the same sound font, so both games’ music and SFX sound very similar.
- The GBA port of Sonic 1, Sonic Genesis, was developed by an uncredited American company. That company? Why, it was Digital Eclipse, who now make all those Capcom classic collections. Neat!
There are probably lots more, but these were the first ones that came to mind without googling.