Continuum-subtracted PACS spectrum of OH 127.8+0.0 is shown in red for the red bands. The PACS band is indicated in the lower left corner of each spectrum. Model 2 in Table 4 with n_H2O/n_H2 = 3 × 10^-4 and ψ_H2O = 0.003 is given in blue. The other parameters are listed in Tables 1 and 2. The colored vertical lines indicate the molecule contributing at that specific wavelength, with full black for ¹²CO, yellow for ortho-H₂O, and cyan for para-H₂O. The dashed black-colored lines indicate the water lines used for the initial H₂O line fitting. The OH rotational cascade line at ~162.9 μm (not included in our modeling) is labeled. At ~ 144.9 μm, another strong line appears both in band R1A and band R1B. A different wavelength sampling causes the line in band R1B to appear weaker, but the integrated line fluxes of both lines are within the absolute flux calibration uncertainty of PACS. This line remains unidentified. The CO line alone cannot explain the observed integrated line flux at this wavelength.