What it does
Walls, floors and doors stop being solid: with Phase a player slips into a wall, drops through the floor or ends up inside a closed room. It usually goes through thin barriers — a door, a one-block wall, a floor.
On anarchy and survival that means getting into other people's bases and bunkers through obsidian; in BedWars, reaching the bed past the defence; in PvP, escaping a box trap. For the server it strikes at the core of the game: builds and block defences stop meaning anything.
How to spot it
- Turns up inside a closed base or room without breaking a single block
- Head or body sticks out of a wall; walks through doors and glass
- Falls through the floor to the level below or pops up from underground
- Leaves a trap without breaking it and without an ender pearl
How it gets caught
A player's hitbox can't overlap solid blocks, so an anticheat checks collisions along the whole path from the old position to the new one, not just the destination. If the path cuts through a wall or ends inside a block, the move is impossible, however many packets the cheat spreads it across.
Honest players end up in blocks too: an ender pearl landing against a wall, a piston, falling gravel, a plugin teleport, a ghost block after lag, crawling (since 1.14 a player can squeeze through a one-block-high gap). Spectator mode passes through walls by design — check the game mode. One case isn't enough; it takes a series.