A ground probe that marks a falling body grounded before contact hides the landing from anything keyed on the airborne flag

Fact

Character controllers often set onGround whenever the feet are within a small tolerance ε above a surface, so walking and jumping stay stable. A body falling at speed v with substep h ends some substep inside that band with probability about ε / (v·h) (capped at 1). The next substep's contact is then handled as a grounded body resting, not as a landing: an impact event guarded by if (!onGround) never fires, and a jump input checked against onGround fires from mid-fall instead. With ε = 0.05 and h = 1/90 s, about 40% of landings from one to eight units up reported no impact.

Why it matters

Landing feedback (a thump, a dust puff, fall damage) is skipped on a large, height-dependent share of landings; being intermittent, it reads as an audio or effects glitch. A bounce pad keyed on the airborne-to-grounded transition loses about half its bounces, and with jump held the standing-jump branch replaces the speed carried down, so a "higher each bounce" mechanic resets at random.

How to apply

• Key impact logic on the contact's own normal speed (faster than one substep of gravity gives a resting body), not on the grounded flag left by the previous substep.
• In the jump-from-ground path, check whether the body is still moving down: inside the probe band it is landing, and that fall speed belongs to the impact.
• Fix the consumers too. A caller that also wants the flag false at the start of its frame (if (landed && !wasGrounded)) re-hides every landing whose early catch fell on a frame's last substep: still about one in five at 60 fps with two substeps a frame, after the physics reports them all.
• Test by dropping from a sweep of fractional heights and counting reported landings; every drop must report one, through the frame loop as well.