Two equal point masses on massless rigid rods, solved with the true coupled equations of motion under gravity — no sine-wave shortcuts. Small changes grow into wildly different paths.
120°, θ₂ = −10° from the downward vertical, ω₁ = ω₂ = 0. Equal masses m₁ = m₂ = 1 and equal rods L₁ = L₂ = 1 m.dt = 1/480 s with a frame-time accumulator (clamped to avoid spiral-of-death). Energy stays bounded, rods stay exactly L long by construction.