It's one of the most annoying errors in the editor.
It's those random bits of invisible collision that are not directly linked to anything and can only collide with vehicles and projectiles pointphysics (like weapons) and ragdolls. There are quite a few of these on the stock maps (theres actually one floating above the overshield hill on bloodgulch) and tons of these on custom maps (especially yoyo).
I made a tutorial on how to remove them a while back.
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