Number of lines overlapping within their thermal Doppler widths as a function of wavelength. (Only lines from the lowest three ionization stages, excluding forbidden lines, were considered for this plot. The overwhelming majority of these lines is from iron-group elements. The thermal Doppler width was calculated for a representative element of this element-group using a temperature typical for SN Ia, and is on the order of 0.01 Å at a wavelength of 1000 Å.) The behavior that up to 70 lines from different elements of the iron-group dominated SN Ia ejecta overlap per Doppler width is the origin of a hidden form of deadlock of the lambda iteration used to solve the multi-level non-LTE problem, and a straightforward lambda iteration will fail to converge under such circumstances unless special measures are taken. This lambda iteration deadlock is a general problem because it is solely based on the fact that a large number of lines overlap within the thermal Doppler widths of the lines, and this behavior cannot be influenced by choosing extremely narrow frequency and radius grids, or by choosing a different procedure to solve the radiative transfer (comoving frame or observer’s frame).