Factual error: In the middle of summer in southern California, water spread thinly on an outdoor ceramic surface would start evaporating immediately and would be gone in about fifteen minutes. The surrounds of the pool would be bone dry by the time Columbo arrived at the scene, nearly an hour after the murderer sprayed the water there.
Plot hole: In the last scene Columbo convicts Hanlon of murder by playing back the recording of Hanlon's final call to the victim Wagner (the phone-box call near the crime scene is Hanlon's alibi. He is pretending to be in his VIP-box in the football stadium, which is too far away at the time he murders Wagner). The point is that the recording is missing the loud clock chimes from a little clock inside the VIP-box, which means Hanlon's alibi is "destroyed", he was not in his VIP-box, he must be somewhere else at that moment. Problem is the missing clock chimes are not hard evidence. Hanlon could say the clock was not working that day or the battery was empty and so on. Beside that it would be much easier to catch Hanlon if Columbo would check the outgoing phone calls asking the telephone company.