After analyzing the milkshakes and leaving, it seems out of character for Columbo to poke his head back in and ask if he was right about the milkshakes and NOT mention the crashing of the glasses when Alex broke them, as he surely must have heard that.
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.
As Lenny and Alex are talking in Alex's studio (after Lenny skips out on the tour at the beginning of the movie), at one point Lenny walks over to a Jukebox that is up against a wall. Meanwhile Alex is standing in the middle of the room. Lenny says "I jumped ship when I saw you". The scene cuts and both men are standing in the middle of the room. No time had passed where Lenny could have walked over to Alex.
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.
When Columbo talks to Alex Brady on the crane from inside his car (at around 1h 14 mins) he first holds his cigar on the left hand outside the car. Next shot the cigar is in his right hand inside the car near the steering wheel. A moment later he holds the cigar again in his left hand outside the window.