Spider

Any wall becomes a ladder: the player presses against stone, obsidian or glass and crawls up without ladders, vines or scaffolding. Some modes climb smoothly, ladder-style; others go up in short bursts.

Players use it to get out of pits, traps and other people's bunkers, scale base walls on survival and anarchy, and reach islands in SkyWars and BedWars without placing blocks. Defences built on tall walls simply stop working against such a player.

• Climbs a bare wall with no ladder, vines or scaffolding
• Goes up without jumping or placing blocks underneath
• Climbs at ladder speed where there's no ladder
• Gets out of a pit trap that can't be left without a pickaxe

Climbing in Minecraft only works on blocks the game treats as climbable: ladders, vines, scaffolding. The anticheat checks which block the player is in while rising and compares the vertical speed with a jump: with no support and no climbable block, you can't gain height by pushing into a wall.

Extra suspicion comes from powder snow (leather boots legitimately let you climb it), a ladder that was just broken, a ghost block after lag, launch pads and wind charges. So conclusions come from repetition, not from a single climb.