Neutron flux obtained on the Flattop-Pu benchmark with the legacy kinetic algorithm, juxtaposed to the evolution predicted by the point-kinetics equations. The transient is initiated by enlarging the internal Pu sphere, and it consists of the maintenance of a positive reactivity insertion of $2.2 for 34 μs on a system initially kept in its critical state for 1 μs. The importance values and importance ratio used in the legacy importance-sampling scheme in this case remain constant, as the legacy algorithm does not support their dynamic adaptation. As a consequence of this less tailored variance reduction technique, the kinetic results from TRIPOLI-4 significantly underestimate the system’s evolution.