Impact of the light curve bump between t_max+240 and t_max+340 days on the SN spectrum at t_max+286.7 days. Top: The spectra before, during and after the light curve bump. Bottom: The observed spectrum at t_max+286.7 days (≈13 days before the peak of the bump) is shown in black. We estimate the “bump-free” spectrum of SN 2018ibb at t_max+286.7 days (red) based on the spectra obtained before and after the bump. The difference between the observed (black) and interpolated (red) spectra at t_max+286.7 days is shown in blue. It reveals a series of emission lines that can be attributed to [O II] and [O III]. An excess bluewards of 5000 Å is also visible, while no apparent residual can be seen at the location of [O I].