This paper published by Lu, Zhang and Zheng in January 2019 reports a rather simple experiment with a rather unexpected and unexplainable result.
Abstract: Here, we report the transmutation of K-Ca under the negative hydrogen condition (NaBH4, LiBH4 and NH3BH3) at room temperature. In all reaction systems, the amount of K+ and Ca2+ concentrations were monitored by inductive coupled plasma optical emission spectroscopy (ICP-OES) and inductive coupled plasma mass spectrometry (ICP-MS) techniques. The ICP-OES test results showed that K+ concentration was gradually decreasing, while the Ca2+ concentration was gradually increasing. In addition, by comparing the K and Ca concentrations and their isotopes from the ICP-MS results, we found the increase of 40Ca concentration accompanied by the increasing concentration of 41K in the presence of hydride under our "reaction" conditions, which implying 40Ca formation correlated to 41K.
I think this experiment is within reach of replication by anyone with access to an academic Chemical Analysis laboratory. I have potential access to replicate this, but I think is only worthy if a proper control experiment can be designed.
If you were going to replicate this, which control would you design? is not straight forward because as the hypothesis is that the H- is the culprit, I see no simple way to use the same mixture of elements to yield solutions with the same dissolved ions and H+ instead of H-. I think it could be more easy to create a solution with the same ions but without H either + or -, but I have to brainstorm with my C.A. friend.
I think the same paper with a proper control would be far more interesting and hard to dismiss. I am just looking for an easily repeatable and unambiguous experiment that many people could replicate. If LENR will ever be accepted its going to take a foolproof replicability and I think this is the closest anyone has got, assuming this can be replicated even without a control.
Adding D2O would also be interesting, but one step at a time.