  $^{75}$As NMR investigation of a single crystal of superconducting LiFeAs is
presented. The Knight shift and the \textit{in situ} ac susceptibility
measurements as a function of temperature and external field are indicative of
two superconducting (SC) transition temperatures, each of which is associated
with its own upper critical field. Strikingly, the Knight shift maintains its
normal state value over a temperature range in the SC state before it drops
abruptly being consistent with spin-singlet pairing. Together with our previous
NMR study, the anomalous SC state featured by the constant Knight shift is
attributed to the extremely sensitive SC properties of LiFeAs, probably
stemming from its proximity to a critical instability.
