Structure elucidators will routinely use data from multiple techniques such as MS and NMR to build a proposed structure(s). When dealing with data from multiple techniques, the issue may arise that the data seem to contradict each other. In these cases, it is best to step back and re-evaluate the data from a different angle.
The ESI+ MS data below shows a prominent [M+H]+ ion at m/z 102 and its sodiated adduct. The maximum number of carbons possible for the ion is 8 (= 102 / 12). The 13C NMR spectrum below shows 12 carbons signals, all aliphatic and no quaternary carbons.
Assuming no issues with the instruments, how the data was acquired or how the sample was prepared, the lingering issue is how can the data from the MS and NMR present such different results for the unknown?