Quick answer: Physics bugs are notoriously hard to reproduce. A test plan that explicitly varies framerate, time scale, and entity count surfaces 80% of them.
Physics bugs hide in the parameters. Vary them deliberately.
Vary framerate
Test at 30, 60, 120, 144 fps. Framerate-dependent bugs surface here.
Vary time scale
Slow-mo at 0.1; fast at 5x. Some bugs only appear at extremes.
Stress entity count
1, 10, 100, 1000 physics bodies. Performance and correctness both visible.
Vary input timing
Hold inputs steady; tap rapidly; combo presses. Input-timing-sensitive bugs surface.
“Physics is parameter-sensitive. The parameters are your test plan.”
Build the parameter-variation test plan once. Each release runs through it; the same bug class doesn't slip twice.