A pendulum's tick is not perfectly even. The famous rule — that its period is 2π√(L/g), the same for every swing — is only true in the limit of tiny swings, where sin θ ≈ θ.
Pull it wider and the restoring pull no longer keeps pace: each swing takes a little longer. The true period stretches by a factor (2/π)·K(sin θ₀/2) — about 0.7% at 20°, but nearly 18% at 90°.
That is why a pendulum clock keeps its swing to just a few degrees: the escapement gives the bob a tiny nudge so the amplitude — and the error — stays small. Christiaan Huygens built the first such clock in 1656.
The gold line is the promise sin θ ≈ θ makes. The brass ink is what the pendulum actually does. Watch where the promise breaks.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.