This page lists various entries that for one reason or another are not accessible during normal Terraria gameplay. Inaccessible entries are categorized as Unobtainable, Unimplemented, or Removed.
Unobtainable refers to fully functional (or partially functional) features which for some reason are unavailable during normal gameplay. Features in this category may or may not delete themselves upon being placed into a player's inventory. They are only accessible through game modification software like inventory editors and tools like Terraria Tweaker, the latter which prevents automatic in-game removal of unobtainable items. Features may also be labelled as unobtainable due to being only partially obtainable - for example, Bone Blocks can be interacted with, and even created during normal gameplay, but cannot be placed in the inventory without using inventory editors. Another example is Moss which naturally spawns in all game versions, but only exists as obtainable Moss Blocks on the Desktop version, Console version, Mobile version, and Nintendo Switch version.
Unimplemented refers to features that are only partially present in the game files. Due to incomplete coding, being replaced by another feature (such as another item), or forced deletion, they cannot be accessed in-game, even with software like inventory editors. The only exception to this inaccessibility is the Boring Bow, which can be placed in the inventory.
Removed refers to features that have been completely removed from the game for various reasons, ranging from copyright liability to former-exclusivity. There is no longer any data related to them present in the game's files, and their Data IDs are typically replaced by something else.
Note that this page does not include features that have never appeared in the game assets of a given version. For example, the Horned God set has only ever existed as assets (and obtainable items) on the Old-gen console version, and does not count as an Unobtainable or Unimplemented feature on other versions despite being inaccessible on them. This means version-exclusive features will not be listed here unless falling under any of the above categories.
In Singleplayer, if you pinch your fingers together, but keep touching the screen, the game doesn't auto-pause and the enemies still move. You can watch how they act with the surrounding terrain and identify what AI type they are.
Yourself, other players and all NPCs will appear as that person's head, with the exception of the Old Man which will appear as a "enemy circle".
Bosses appear as a large skull icon, except for the Eater of Worlds which appears as lots of monster circles. The Destroyer is a lot of enemy circles with a skull for the head.
Worm-type monsters appear as lots of medium "enemy circles" in a line, due to each body segment being classified as a new enemy.
Tapping on a player/NPC's head icon will display their name.
If you or someone else is attacked with the minimap open, the unmapped spots on the screen will flash red.
Short, dynamic nights in pre-Hardmode. In order to accommodate the shorter play times that mobile game sessions tend to entail, day and night last for shorter overall periods, and boss or event fights can cause the day or night period to extend in order to offer players more time to complete them. However once Hardmode begins, the day and night cycle will progress at the same speed as in other versions. Pre-Hardmode daytime is 15 minutes whereas nighttime is 9 minutes.
Note: The Ocram Mask should have been obtainable on the pre-188.8.131.52 Mobile version (which includes Windows Phone), but is unobtainable even with inventory editors (info taken from Ocram page). It is unknown if this means it was never added to Mobile at all.
Icemourne will have the description of the Boreal Wood Work Bench, and Scythe will have the description of the Boreal Wood Table, but the sprites will remain as their respective original sprites. Both have their swing animations but no damaging hit boxes, and may also place as Angel Statues.
All Developer items edited in on the version will have the tooltip "You shouldn't have this" and will disappear upon selection (no button to wear the vanity set, no matter how much of the item, will always say "0 items in inventory") except for Red's Wings (part of Red's set).
The Golden Seaweed and Shiny Black Slab items are presumably obtainable on the version through trading from the Amazon Fire TV Mobile version and Android Mobile version, respectively. Both items are technically unobtainable as they were removed from those versions.
Power-up items (Heart, Star, their derivatives, Mana Cloak stars, and Nebula armor boosters) all appear during normal gameplay, but cannot be added to the player's inventory without inventory editors. However, unlike items like the Bone Block, which well could have been a storable item, there would be little point in storable power-ups as their purpose is to provide spontaneous benefits. In this sense, power-ups are not unobtainable items.
The unobtainable weapons are not available even with an inventory editor. The game must be edited to prevent it from deleting the items from the inventory.