The police photo of Ellis that Perez shows to Mike and Steve is taken directly from the footage of him in the earlier scene at the bar, with the same woman in the background, possibly because the film editor had no other photo to use during post-production.
When Stahlbecker holds a woman hostage at knife point, in order to appease him,
Keller gives up his gun by putting it on the center of a busy escalator and letting it slide down away from him. Not only are police trained never to give up their guns, but Keller's doing so in a way that makes the gun accessible to passersby would be even worse.