Given any experimental data, producing a list of scenarios that are consistent with the data can help guide an elucidator through the problem while ensuring nothing gets overlooked.
For an unknown compound, the carbon count is identified at 18 in total. The 13C NMR spectrum below indicates 19 carbon atoms. There are two 13C signals, 29 and 147 ppm, which do not show any correlations on a 1H-13C HMBC spectrum (not shown). The 1H integral information shows one carbon as a C(q) (147 ppm) and the other as a CH3 (29 ppm). Can we rule out either signal as an impurity?
Note the 1H count from the 1H NMR spectrum (not shown) will not help as there is interference from the impurities.