There was a glitch I knew about in Sonic 3 & Knuckles that would causes animated tiles to glitch out and look like eyeballs looking around. Unfortunately, I can no longer replicate the glitch and I don't think it can be found on the world wide web. It was so very complicated to pull off that I doubt your friend saw this. There are these, but their from Sonic Spinball and are hardly related to Halloween. I don't even think it's possible to see them normally either but I heard that some people find these creepy: Spoiler Actually I would consider these infernal; the death sound rapidly plays over and over and weird unreadable text strings appear in the text box above. I also just remembered this bug: It requires debug mode though. Spoiler Sorry for large image size, some devianart guy took the screenshot.
So you expect all Latin Americans to refer to Tails and Eggman as "Colitas" (little tails) and "Mostachón" (big moustache) respectively because that's what they were called in their region? Not necessarily. You can have a different surname from your grandfather and cousin. Regardless, Sega's current stance is that Eggman is a nickname, and his real name outside of Japan is Ivo Robotnik, while his Japanese character bio states that his real name is unknown. Actually, they both have the surname Robotnik in Japan as well. They're a rare instance of a localization name carrying over to the original region.
The swinging platform at the start of Mystic Cave Act 2 detaching from the chain like in Aquatic Ruin.
Misremembering: Sonic 3D Blast on the Saturn looking and playing like low-frame garbage. :v: I'm not sure what I was thinking of these past few weeks as it looks fine in footage. Maybe I had poor settings on my tv when I played it. Although no doubt the Saturn controllers would've messed me up a bit. Only had the 3D controller with nice D-Pad but weird-feeling buttons and a regular controller with wonky-ass D-Pad and nice buttons. Either way I was screwed.
This last one seems related to what my friend saw, since Sonic is a bit ".exe". It still doesn't show a halloween or whatever level, so he could even be talking about more than just one bug; as I said before, some kind of weird palette swap. The "eyes" bug you mentioned sound quite appropiate, but I think you're right about him not seeing it. I still think he could have seen an early hack and be misremembering this, but at least I can show him the "hellhog" bug, so thanks for your time and effort.