Axial gauge has been used in many perturbative calculations. However, it sometimes leads to puzzling results. In this talk, I will discuss a specific example in the context of heavy quark and quarkonium transport coefficients, where naive application of temporal axial gauge leads to an incorrect result. Then I will discuss the origin of the problem from both perturbative and nonperturbative aspects. Finally I will summarize the applicability condition of axial gauge and give a few examples.