Essentially, You're only allowed to touch/stand on solid ground, below I've listed the things me and SmallAnt1 found that should be illegal.

For clarification, no standing on/in, swimming, taking damage or bouncing off of anything below. If clarification is needed please contact me.

No Water, Lava, Poison Water, quick sand, falling off the map, leaves, awnings or umbrellas, bouncing off vehicles, Floating Platforms or moving platforms that are not physically possible, including the flower paths.

Water - No standing in water or swimming in water, the only excusable water is the puddles in Metro when its raining.

Lava, Poison Water - You can not touch it, no death warping off of it.

Quick Sand - No touching or standing in it as you sink and its not solid, if you go for the holes that are surrounded by it you must swish, no net.

Falling off the map - You die and don't touch land, so its the end of the run, exception is if you warp to the Dark Woods.

Leaves - These mainly include the ones in Lost world, but apply to any trees.

Awnings/Umbrellas/Vehicles - They are not solid and cause you to bounce up, you may not touch any of these, you MAY ride the scooters or capture the taxi that allows you to.

Floating/Non-Physics Platforms - No riding on any of these, exceptions are if they are the entire level (Cloud Kingdom or the Rocket area in Wooded Kingdom) or if they obey physics/can be argued with physics. (Conveyer Belts, Attached to the wall, machines). You must flower skip.