The most common unsolvable situation where 2 objects of same size are aligned (same y) is already treated separately (see above). There could be other cases (such as for two or several extended objects which are almost aligned) in which weights along y-axis happen to add up in such a way that the algorithm to separate the spectra fails. Two examples are given in Figure 3.7. Note that in this situation, there is no information in the image which could be used to decide to which spectrum a particular feature belongs. Therefore, no algorithm can solve this problem.
In the particular algorithm used by the software, this will show up as a non-invertible matrix. Such a case has to be noted. The fact that there is one such situation on a grism image should not prevent other pairs of spectra to be decontaminated. Therefore, one has to search for sub-matrices which can be inverted in such a case.