From Sonic Retro
This is the bug list for Sonic Advance.
Upside Down glitch
This can happen by accident with any character but only rarely; with Knuckles it can be controlled. In the levels Egg Rocket Zone and Cosmic Angel Zone, there are areas where the gravity reverses, making the character move around upside down. Using Knuckles to control it can lead to some very weird shenanigans, placing him upside down in the wrong areas.
In Egg Rocket Zone, it's easy. When Knuckles encounters the first area that places him upside down, he must simply face to the right toward the vertical switch point, jump straight up, fall toward the ground, and begin a glide just before landing (so he skids right into the switch point). His gravity will reverse during the skid, and if he turns around and goes back where he came from he will stay upside down.
In Cosmic Angel Zone it works differently; he must go right from the entrance, then straight down into the upside down area below. There is a wide area just above the death pit where he can glide back and forth; since the vertical switch point is halfway through, he can glide infinitely, changing gravity whenever he gets too close to the bottom. As he glides, his speed increases until he glides off the screen (but be careful, for his speed will reset if he hits the left boundary of the level). Once he's picked up dangerous speed, carefully get him close to the bottom of the screen, then go to the right and keep heading that way even after he disappears from view. Occasionally at this point, he may fall right out of the air and die during the transition for an unknown reason; more often however, he will eventually reach a climbable wall (still upside down) and he can enter the middle of the level from here.
Alternatively, in either Zone he can also trigger this glitch by climbing down through a corner while upside down. He will not get off the wall and onto the ground, but rather he'll climb down through the corner and fall through the wall. This usually can kill him in Egg Rocket Zone, but it's safer in Cosmic Angel Zone.
Not fixed in any version.
Path swapper glitch
Players using Sonic, Knuckles or Amy can crouch near certain loop-de-loops with their back to them, wait for the camera to go down and move with the camera locked on that position, causing the map to lose its solidity. This can be reverted by triggering a path swapper, which is not easy to do because the ground is not solid as a result of the glitch. This glitch is useful to time attack Angel Island Zone Act 1 as Knuckles, and it can be seen in this video.
Not fixed in any version.
Tiny Chao Garden glitch
The Tiny Chao Garden mini-game has a major glitch. If the game data is deleted, rings obtained in the main game no longer transfer to the mini-game. There are two ways to revert this; using a video game enhancer such as GameShark or collecting the same number of rings that had been collected before the game data was deleted.
Fixed in v1.1, released only in Japan.
Broken Multiplayer Mode on startup
Sonic Advance uses some of the link cable registers to determine if it's running in multiplayer mode. Some older emulators don't implement this correctly, which can result in multiplayer mode loading on startup with blank player slates. Soft-resetting (+++) can usually get to the main title screen.
Fixed in newer emulators.