If a bsp brush is deleted from a level, the level has geometry built, and the user presses play then uses the "show collision" console command, the collision for the bsp wlil still appear. This does not affect gameplay and does not produce actual collision.
1. Open Editor (any project)
2. Place bsp sphere in level (Modes>Placement mode>BSP>Sphere)
3. Delete sphere
4. Build Geometry (Build>Build Geometry)
5. Press PIE
6. Press ~
7. Type Show Collision, press enter
Collision for sphere appears in level where sphere was placed.
Collision for objects still in level appear, the sphere is no longer present.
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