Concept art features multiple color variants for SPIDER. In the final game, the white variant was recolored to red and used in Chaos Island.

FORTRESS's design resembles a Seraphim, part of the highest order of angels in Christianity. This design is heavily associated with the term "biblically-accurate angel".

For the Adventure's Gloves and Shoes costume DLC, the vertex groups on Sonic's mouth are incorrectly weighted, producing a glitched appearance. This doesn't happen on the default model.

Submitted by Asurilm

Tails' CyBlaster is a modified Drill Wispon from Sonic Forces.

Left: Drill Wispon, Right: CyBlaster

This piece of concept art seems to be a paint-over of an in-game screenshot of 4-2. Compared to the final, it has different lightning, different sun angle, more glitch effects, different text on the signs, and cars on the far roads in place of light trails.

STRIDER's design is based on a bacteriophage, a kind of virus that infects bacteria.

The eastern portion of Chaos Island has a small error in the heightmap, causing this piece of terrain to jut out.

The Ancients' pinball table is set up incorrectly. A proper pinball table is at a slant to ensure the ball behaves predictably, but the Ancient's table is straight up.

I don't think the Ancients played a lot of pinball.

In Ares Island, Sonic and Knuckles comment on the challenge indicators, suggesting that the symbol's true meaning is "lost to time".

The symbol is... a question mark.

Cyber Space 1-3 is the *only* highway level with buildings!

Most "dead end" spots in #SonicFrontiers have springs that prevent you from getting stuck or softlocked. Here's one of them, situated on the farthest hexagonal pillar on Chaos Island.