In Skyblock there are hundreds of different glitched items. There are few main categories that most glitched items fall into, each obtained in unique ways and from different sources.
Nulls[]
The first kind, and likely the most notable, are Nulls
Nulls are made when an item is given a damage value it normally isn't supposed to have. When Minecraft detects the value of this item, it does not now what texture to pair with it, because the block/item is not supposed to exist. Whenever this happens, Minecraft is coded to apply the purple and black null texture to the block.
GUI Items[]
The second category of glitched items are called GUI Items
GUI stands for graphical user interface. GUI items are items that are stolen from interfaces like NPC's menus, the Skyblock Menu, and other menus within Skyblock. Most will have a custom name and lore(this is special because you cant rename items using the anvil in Skyblock).
Glitched Heads[]
The third category of glitched items are Glitched Heads
Many items in Skyblock use player heads to give them a unique look. When one of these player heads is placed down it is considered a glitched head. That's because those Skyblock items aren't meant to be placed down.
Outdated Items[]
The fourth category of glitched items are Outdated Items
Outdated Items are items with outdated an texture, name, rarity, and/or lore. These happen when you keep you items in a chest. If the admins decide to change the texture, name, rarity, or lore of that item the older version will stay if it is in a chest.
Miscellaneous[]
The fifth category of these items is Miscellaneous
Many glitched items just don't fit into the other categories, but aren't unique enough to make their own category.