Minecraft wiki entry for Debug_mode, associated text: Water and lava cannot be removed by buckets, but still fills an empty bucket in Survival mode. If the player empties the bucket, the lava or water blocks turn invisible.
Bells can still be rung.
Cauldrons clean leather armor and banners, but cannot be filled by buckets.
Using a button or lever drops one of itself as an item.
Activating those blocks schedules a block tick for its own block, which drops one of itself because those blocks cannot be suspended in mid-air.
Warning: using a pressure plate with powered stated false, or a weighted pressure plate with power state 0, causes it to generate enormous quantities of dropped plates, likely crashing the game. It also produces an extremely loud sound from so many pressure plate sounds being played at once.
The pressure plate schedules a block tick for itself, and drops an item because it cannot be suspended in mid-air.
Since it never changes to an activated pressure plate, it never stops receiving activation from the entities and/or items on it, and never stops scheduling block ticks.
Certain pressure plates, when activated, cause an error in the game output, stating that the dropped item entity has no entity.
Using a comparator in subtract mode, a lever or an unpowered button causes the block in the direction opposite its facing property to drop as an item.
If that block is a repeater or a comparator, it drops 36 stacks of itself.
It starts to repeatedly schedule its own block-state-changing tick, which fails continually a large number of times.
These types of blocks schedule a block tick in the adjacent block.
If that block could not normally remain suspended in the air, it drops one of itself as an item.
Warning: placing a minecart on an unpowered detector rail drops an infinite number of detector rails, crashing the game.
Certain glass panes, iron bars and fences have incorrect hitboxes.
Any summoned falling_block entities drops as an item when it hits another block, rather than converting itself to a block. Such a summoned falling block immediately drops as an item, rather than fall any distance.
Using fence gates, comparators, deactivated buttons, levers, deactivated pressure plates, doors, or trapdoors does not move or change the block, but the sound still plays.
Nether portals are visible, but the End portal is not.
Shulker boxes are invisible until looked at.
Using a music disc on an empty jukebox does not change its block state but still plays the music normally.
Firework rockets do not boost the player while flying with elytra. Instead, the rocket flies upward as if it was used while looking at a solid block.