Induced two-photon decay of the 2s level and the rate of cosmological   hydrogen recombination
  Induced emission due to the presence of soft CMB photons slightly increases the two-photon decay rate of the 2s level of hydrogen defining the rate of cosmological recombination. This correspondingly changes the degree of ionization, the visibility function and the resulting primordial temperature anisotropies and polarization of the CMB on the percent level. These changes exceed the precision of the widely used CMBFAST and CAMB codes by more than one order of magnitude and can be easily taken into account. 
J.Chluba and R.A.Sunyaev