A character I'm thinking of is about to have a close call. A woman attacks him with a stun gun by surprise, but by chance he is able to catch the weapon. Backup operatives move to assassinate him conventionally, but he is able to escape by an unexpected route.
The stun gun appears to be a model sold by a small company (i.e. it should generally look like a stun gun carried semi-legitimately for self defense) It is, however, a counterfeit - the woman planned to exchange it with another agent, so she could present a verifiable stun weapon to police if speaking with them became necessary. An agent would also testify that the victim made an inappropriate sexual contact just beforehand. The site was chosen for lack of mounted cameras, and routine 5G terahertz scanning of the scene and weapons by passerby smart phones has been blocked during the incident by "chance" interference.
On examination, the weapon is designed to kill reliably when applied to someone's heart by a trained user. It does so without leaving evidence differing from the stun gun it mimics. The question is whether (or how) this is done. A few minutes searching turned up this abstract explaining the electrodes should go above and below the heart, and work by controlling ventricular rhythm with pulses of electricity. When these steadily speed up, presumably the victim would be driven into fibrillation.
This is not quite the entire story, however, because several other articles claim humans are more resistant than pigs (the electrical signals in the heart run deeper) and the voltage of a Taser is only half of what is needed to control the heart rate. The temptation is to double the voltage, but this assumes everything goes perfectly - yet still leaves more severe surface markings. Some other trick may be needed.
I would be surprised if such weapons have not already been used in the field by certain unsavory agencies that closely follow weapon development, yet I'm also not entirely sure they're possible at all.