Columbo spots a couple of ice-cream sodas left on the counter in Alex Brady's office, known as his "boys' club". One glass is almost full, the other about half-full. Not only can we see this, but Columbo comments on it at length, theorizing with uncanny accuracy how the full and the half-full glasses show what happened earlier, when the victim was visiting Brady's office. The moment Columbo leaves, Brady rushes to the soda glasses, to destroy the evidence. Inexplicably, he finds both glasses are now empty except for a milky residue.
When Columbo is eating at the commissary and overhears the conversation between the two actresses, he puts ketchup on his food. But, when the camera switches to a two-shot of the actresses, the ketchup bottle is sitting on the counter in front of them.
When Columbo tells him the victim is his friend Lenny, Alex puts his glasses up to his forehead. He later moves the glasses back on his nose, but in the next shot from behind the glasses are still on his forehead.