Plot hole: Lupin was knocked out by Pycal when he shot him with a gun at point-blank. On the other hand, the 'shield' he used was strong enough to not even budge when Jigen's gun and several machine-gun shots would hit, and sustained the impact of an anti-tank rocket, so the dampening powers of the chemical are pretty inconsistent. It's also completely impossible that it works as shown, unless it also coats the mouth, the nostrils, the eyes.
Plot hole: Lupin was captured by chance, and he did not plan for that, nor he seemed to have previous knowledge of Stern's appearance, but he escapes regardless using a Mission Impossible-like perfect mask of him. He also somehow kept his own blazer, tie and shoes underneath the uniform and boots he stole and wore. (00:16:00)