House full of cats where they were "marking" one of the receptacles. Eventually enough current would flow to trip the 15 amp circ breaker. Saw no dead cats.
I had that, in an older house. You know, the kind originally wired in K&T, with BX and NM additions over the years, and a fairly recent breaker-panel upgrade. At least there were no fuses to deal with.
There was an intermittent short in an added circuit feeding various added first-floor receptacles. I used a 100w bulb in a rubber socket in series with the circuit at the breaker as a troubleshooting aid.
With everything unplugged, the bulb would continuously vary in brightness, sometimes full-on, sometimes completely off, usually in between. I narrowed it down to a receptacle in a Wiremold box.
With the box cover off, I could see a varying arc inside the receptacle, with the smell of burnt cat urine wafting out of it. A new box and receptacle later, everything worked (and smelled) as it should.