Consider a smooth projective curve formula_108 over formula_73 of genus formula_110. Let formula_111 be the moduli stack of stable vector bundles on formula_108 of rank formula_113 and degree formula_114. It has a coarse moduli space formula_115, which is a quasiprojective variety. These two moduli problems parametrize the same objects, but the stacky version remembers automorphisms of vector bundles. For any stable vector bundle formula_116 the automorphism group formula_117 consists only of scalar multiplications, so each point in a moduli stack has a stabilizer isomorphic to formula_75. It turns out that the map formula_119 is indeed a formula_75-gerbe in the sense above. It is a trivial gerbe if and only if formula_113 and formula_114 are coprime.