Plot hole: Unless Jarvis Goodland has the world's most stupid lawyer, he has nothing to worry about. Columbo searches the greenhouse and uses a metal detector to find the bullet which, supposedly, shows Goodland's gun is the murder weapon. He conducts the search without a warrant and against Goodland's wishes - he asks him several times to desist with the search and leave. Any evidence discovered or collected during the search is inadmissible in court.
Shootout at Diablo Station - S2-E11
Plot hole: Well within earshot of the others in the small room, Curry comments to Heyes (purely for the viewers' benefit, since Heyes already knows it) that if Lom Trevors is killed, their amnesty will go with him. This information should have blown their cover, but for some strange (and unbelievable) reason, no one wonders what he's talking about. (00:19:00)
Farewell My Beloved Witch - S1-E3
Plot hole: Lupin was captured by chance, and he did not plan for that, nor he seemed to have previous knowledge of Stern's appearance, but he escapes regardless using a Mission Impossible-like perfect mask of him. He also somehow kept his own blazer, tie and shoes underneath the uniform and boots he stole and wore. (00:16:00)