The one bit of trivia as far as Columbo background wall objects are concerned grabbed my interest in "Forgotten Lady." As Columbo is in the doorway talking to Ned Diamond, one of the paintings from "Suitable For Framing" is on the wall.
And, of course, the portrait of Mrs. Melville from Murder by the Book reappears in The Bye-Bye Sky High IQ Murder Case. That's not a goof, though, since it's entirely possible Joanna Ferris might have sold the painting at some point. Could be that after her husband was murdered by his pseudo-collaborator, she never wanted to hear the words "Mrs. Melville" again. Who knows, maybe she sold it to Oliver Brandt and thought, "What a nice man -- too bad Ken couldn't have been more like him!"
god bless all of you...ive only spotted one goof so far and then saw it on this webpage...the one in etude in black at the end where the wifes hair is up in a bun and when she goes in the studio, her hair is in a ponytail...im guess im not very observant!!! ill keep trying
there is also a weird hair thing in "old fashioned murder." when columbo meets janie in the living room with ruth, her hair is very curly. in the next scene, ruth takes janie to the museum to do inventory (and have her discover the body). janie is wearing the same clothes, but her hair looks very different--much straighter.