There are some sign ambiguities arising naturally from the geometric Langlands conjecture (GLC) and its quantum version. These issues can be easily suppressed initially, but they get increasingly subtle and interesting. Therefore, it is essential to view these ambiguities as an intrinsic part of the GLC, i.e., spin structures in GLC. In the first half of the talk, we discuss some examples of the sign phenomenon and draw analogies to spin geometry. In the second half, we push this analogy further to give an interesting viewpoint on spin geometry inspired by a construction in GLC.