I'm posting some glitches for the heck of it.
Sonic Adventure (DX)
In Hot Shelter where you drain the water, I did so and hopped on the platform that leads you to the next area. I did a spin hammer on the platform as the water was rising and I fell right through it. To prove that this only happens when you spin, I didn't fall through when I didn't spin.
Sonic Adventure 2 (2 Battle)
Floating in the Loop
So on 2-player in Battle, I used storming heart as Amy on my friend in Radical Highway. He was on one of the loops where you end up on some girders. When he was hit he floated up (similar to what happens when you fall into that large gap in said stage) for quite awhile until he hit the top and fell.
Infinite Jump on Water
So I was in Tropical Jungle as Sonic, and I had just fallen on the water of which you can't escape. After the aura around me was gone I kept jumping looking for an exit (which I couldn't find) until I did a homing attack and fell in the water.
Under Solid Water
So I went into that Chaos mode with a full meter with Shadow did several automatic homing atacks on the Ibis and Mephiles (I call them that for their purple color) enemies in Aquatic Base and somehow I fell through the solid liquid floor and died.