Exactly. We'd like to think Columbo would never have gone through with trying to use the planted pearl in a court case, but fortunately he didn't have to, because his plan worked -- using the fake evidence to drive Frame over the edge and into a raving confession.

The giveaway is right in the title, "Dagger of the Mind" -- a reference to the speech in Macbeth (the play they were doing, remember), where MacBeth goes mad from guilt overer a murder, and starts hallucinating. "Is this a dagger that I see before me, the handle toward my hand?...or art thou but a dagger of the mind, a false creation,
proceeding from the heat-oppressed brain? "

You guys are quite right. I knew Columbo was counting on Nick's meltdown, but to hinge it upon a gaff that could just has easily have backfired was taking a mighty big risk. I mean, not even Durk knew what Columbo was up to.

One thing we need to keep in mind regarding the questionable admissibility of this is that it likely isn't going to matter because it's clear that there isn't going to be a trial. Lilly clearly plans on making a statement that's going to result in a guilty plea deal.

That may in fact be true of some of the other cases where we've said the killer would get off at trial because the evidence wouldn't hold up in court. We fail to take into account the fact that the killer is likely at episode's end taking the "I'm going to make a statement and get the best possible deal from the DA I can" approach. That would likely apply to Brimmer ("Death Lends A Hand") and a few others.