Java Edition:Invalid Projectile Crossbow
Throughout the history of crossbows, there have been multiple ways to load normally unobtainable projectiles into a crossbow.
Obtaining
Air Crossbow
From 18w43a (crossbows added) to 1.14.1 Pre-Release 1, the player could load a crossbow with "Projectile: [Air]".[1] To obtain, the player must have the crossbow in their offhand and an arrow, tipped arrow, or firework in their main hand. The player must not have any other projectiles in their inventory. The player then needs to begin drawing back the crossbow. In the middle of the loading process, the player needs to drop the item in their main hand. Once the crossbow is done loading, the projectile will be set to air.
Empty Charged Crossbow
During The April Fools version 3D Shareware v1.34, loading a crossbow that does not have the Multishot enchantment charges it, but doesn't actually load a projectile. Attempting to hover your mouse over the crossbow will crash the game (the crash also occurs in snapshot 19w14a). Updating past 19w14a will give you a crossbow that appears charged, but is missing a projectile. It functions in the same way as an air crossbow.
Too Many Projectiles
Loading a crossbow with Multishot 2-12 in 3D Shareware v1.34 loads more than the usual amount of projectiles onto the crossbow. The amount of projectiles loaded is based on this simple equation: Projectile Count = MultishotLevel * 3.
Overstacked Projectiles
From 18w43a (crossbows added) to 24w20a (present), loading a crossbow stores the count of the stack the projectile was drawn from. This means the stack could be saved as overstacked as well. The player may need to bring the overstacked projectiles from older versions. The most robust way is to store overstacked items inside inventories, as the player can use the 1-9 hotbar and offhand swap keys to swap the overstacked stack into the player's hotbar and offhand without destroying the overstacked item, but it is impossible to place items with counts more than 64 inside inventories in versions 20w48a (a 1.17 snapshot) and later, making this method only useful for storing items available in older versions. The crossbow stores the stack size and then decreases it by one.
Normally Unobtainable Projectiles
Crossbows can load any firework or arrow, which includes unobtainable variants. Below is a list of these unobtainable projectiles:
Observability of Different Projectiles
As the NBT storing the projectiles list is not completely available to the player, these crossbows have varying observability. Air and empty loaded crossbows are directly observable by the item's tool tip, which displays the item's name (including renamed items), and texture, which will show if the crossbow is charged. If air is loaded in the crossbow, it is directly visible in the tool tip, and if nothing is loaded, then the tag will be missing, despite the crossbow being charged. The other projectile tags are more difficult to determine without the usage of commands. The only in-game test method available is via the ⇧ Shift + stack in cursor + double left click inventory shortcut, which will move all items with the exact matching NBT of the double clicked stack between inventories. The stack in the cursor does not matter. With this test, one needs one of each normally possible crossbow for the type of projectile loaded, if it can be determined. For crossbows loaded with arrows, spectral arrows, or tipped arrows, there are 64 possible item stack counts. For crossbows loaded with firework rockets, there are an approximately 2x10^136 possible firework rockets. If the item has been renamed, then all projectile types must be tested. Due to the sheer amount of possible firework rockets, it is impractical to test crossbows containing them or renamed items (as it cannot be determined if the item isn't a firework rocket). If the crossbow being tested is not moved by the inventory shortcut with any of the normally possible crossbows, then it is proven to be discontinued, but its specifics cannot be precisely determined without the usage of commands or external tools. Therefore, these crossbows are indirectly observable.
The exact amount of projectiles loaded in a crossbow may be learned by firing it in any version, but doing so will destroy the item, making it a semi-observable testing method.
Gallery
An empty charged crossbow
