Example of a system from the 20flat suite that experienced a post-disk instability. Left: evolution of the orbital elements and masses over time. Disk dissipation is marked by a vertical line at 5.1 Myr. The horizontal dashed line in the top left panel marks the planet trap at 0.1 au representing the disk inner edge. Black Xs mark when planets were removed from the simulation after a collision and merger. This system clearly has distinct outer and inner chains separated by a period ratio gap of 3.45. Right: mean motion resonant angles of adjacent pairs of planets in the system from out to in, with resonances determined from the state of the system at the time of disk removal. The y-axis in each panel ranges from 0 to 2 π. The x-axis is scaled linearly. All pairs except the planets across the gap are in a librating resonant state at the time of disk dissipation, but the libration amplitudes of the outer chain increase over a timescale of ∼ 100 Myr, eventually entering circulation and triggering an instability. It appears that this perturbation was passed to the inner chain, making it unstable.