The most likely meaning is a Roblox phase-through-walls glitch : a bug or exploit where a player or part clips through another object instead of colliding normally. Roblox forum posts describe similar “phasing” issues caused by collision handling, positioning, textures, or precision limits, and some posts also discuss scripts that let characters move through geometry.

What it usually means

  • In building systems, it can mean a placed part “phases” through another part when you expect it to stop.
  • In movement or combat games, it can mean a character clips through walls or the ground after a phase ability or glitch.
  • In some cases, what looks like phasing is really a rendering or texture issue rather than a true collision bug.

Why it happens

Common explanations mentioned in Roblox discussions include:

  • Collision/placement logic that is too loose, so parts overlap when the cursor gets close.
  • Precision or floating-point limits, where very small spacing differences stop behaving as expected.
  • Textures, decals, or visual layers making parts look like they are colliding or passing through each other when the actual hitboxes are different.
  • Scripts that directly set position or CFrame, which can push a character partly into the map.

Practical fix ideas

  • Increase spacing slightly between parts.
  • Check whether a decal or texture is causing a visual illusion.
  • Review collision settings and placement code.
  • For scripted phasing, clamp movement so the character stops at the nearest obstacle instead of moving the full distance.

Context

This topic is often discussed in a “glitch” or “exploit” context, so the meaning depends on where you saw it. In a game-building context, it usually refers to a collision bug; in a player-movement context, it usually refers to wall phasing.

If you want the cleanest one-line definition: “X mark part phase glitch” most likely means a Roblox bug where a part or player phases through something that should block it.