Java Edition:Floating Gravity Block
Gravity effected blocks are blocks that require a block to stand on, otherwise, they'll become entities and fall until they hit a solid block. Some bugs allow a gravity block to float without the need of any supporting blocks.
Obtaining
Classic Liquids
From Classic 0.0.14a to Classic 0.0.22a_05, sand or gravel fall on top of water or lava as if they were solid blocks instead of falling through them. In Classic 0.0.19a (sponges added) and later, you can also make sand or gravel float in the air by drying up the water below it with a sponge. The sand/gravel will not update and will remain floating, but breaking the sponge will update it and cause it to fall.
No Fall On Place
From Indev 20100110 to Indev 20100207-1101, simply placing sand or gravel against a wall would leave it floating in the air. The gravity blocks does not fall when placed but will fall if updated. The player can get sand or gravel in the air, not touching a wall, by placing it on an instant mine block, such as a torch, at the same time that block is broken.
Scheduled Tick Interrupted
From Infdev 20100618 through Beta 1.9 pre-release 5, if you place a gravity block while entering the menu at the same time, the gravity block will remain where it was placed after exiting and re-entering. This method also allows you to place blocks next to the gravity block, simply by repeating the same process of entering and exiting the world each time you update the block.
Signs
By letting a gravity block fall on top of a sign, it'll keep bouncing on top of it. Placing another gravity block on top of the bouncing block and reloading the world will cause the top gravity block to remain suspended. This works from version Infdev 20100618 to Infdev 20100630-2.
This method is not consistent, and may have to be attempted multiple times.
Floating Point Error
For every other block from 2^23 blocks out to the world border, if you place a gravity block the block will not fall. This works from Infdev 20100618 to 14w31a (1.8 snapshot).
Generation
Sand, gravel, and red sand all generate naturally in the world. When a cave, ravine, lake, or even just a cliff turns the block underneath them to air they will not automatically fall. This bug is however limited to where these blocks generate, no way to get it in other areas of the world. It's also the only intended method of obtaining floating gravity blocks.[1]
Update Suppression
Using update suppression, the player can remove the block underneath any gravity block and it will not update the block. This works from Beta 1.6 Test Build 3 to 1.21.6 Release Candidate 1 (present).
Tall Flowers
By placing a gravity block on top of a tall flower and breaking the base of the flower, the gravity block will remain suspended.[2] This works from version 13w36a to 1.13-pre2.
After 1.13-pre2, tall flowers are treated as blocks that gravity blocks can fall through, so just updating gravity block on top of a tall flower will make it float.
No Update Piston Retraction
By placing a gravity block on top of the extended arm of a sticky piston attached to a slime block that has reached push limit, then retracting the piston arm the gravity block will remain suspended until updated.[3] This will also suspend any item that requires a block to stand on and can be placed on top of an extended piston arm, such as water, lava, sea pickles, carpets, banners and signs, but not lanterns and bells. Works from 17w49a until 1.21.6 Release Candidate 1 (present).