A capsule end that stops flush with the wall it joins leaves a notch that traps a body for good
Fact
Collision worlds are commonly built from capsules — a segment plus a radius — because one primitive covers walls, rails, guides and gates. Where two of them meet, only one arrangement is safe: the joining capsule's endpoint must sit inside the other's body.
An end that stops flush, or overshoots by even a few pixels, leaves a small upward-facing shelf between the two surfaces. A body that lands there is pushed by one surface into the other, and every individual resolution is correct, so it never leaves. The same shape appears wherever a member crosses a wall and pokes out the far side.
The related failure is a passage whose clearance is the sum of two radii plus the body's diameter and comes out a fraction short. Nothing reports it: the route is simply never taken, and the symptom is "the physics feels wrong over there", several layers away from the arithmetic.
Why it matters
Neither defect is visible in a screenshot, and neither is reachable by ordinary play — a body has to arrive slowly and from one particular side. So they survive manual testing and appear once, to somebody else, as a permanently stuck object.
Because the collision code is correct, the investigation goes to the resolver, the timestep and tunnelling before it goes to the geometry.
How to apply
- Bury the joint: end the joining capsule on the other's spine, not on its surface. Overlap is free; a stub is not.
- Order members by height where they cross: nothing may protrude above the surface a body slides along, or the protrusion becomes the shelf.
- Verify by sweep, not by eye. Release a body from a grid of positions across the whole world, step for tens of seconds, and assert every one resolves — drained, captured or still moving. Print the resting coordinates of the ones that do not; the trap's location falls straight out.
- Keep a last-resort recovery anyway: shake anything that has been stationary for several seconds, excluding states where being held is the intent.
Related: [[boundary-clamp-runs-after-collision]], [[forced-input-obstacle-fits-the-band]].