Non-Abelian Stokes Theorem and Computation of Wilson Loop
  It is shown that the application of the non-Abelian Stokes theorem to the computation of the operators constructed with Wilson loop will lead to ambiguity, if the gauge field under consideration is a non-trivial one. This point is illustrated by the specific examples of the computation of a non-local operator. 
Ying Chen, Bing He and Ji-Min Wu