Once in a great while you notice the opposite thing, where something that SHOULD be a clue gets overlooked (though I guess sometimes that's because of editing). It's always bothered me slightly that Adrian Carsini buys a $5,000 bottle of wine, knowing that his $5,000 check to Ric won't be cashed. It's too bad that Columbo isn't shown finding out about that - even though he finds out that a bottle of wine can go for that price, I don't think he learns about that particular one. Or does he?
Good point. Along those lines (things that should have been clues), I always think of "A Friend in Deed."
Hugh shows up at the club, clearly nervous or upset, and pulls the police commissioner away from the gaming table to have an urgent private conversation. Later that day, his wife is found murdered. The murder is important enough for the police commissioner to go on television talking about it, yet somehow nobody remembers or reports that strange meeting between the murder victim's husband and the police commissioner on the very day she was murdered.