Just spent way too long getting this bug and not knowing why. After figuring it out, here's the simplest repro I could make

it('Mocha is confused', function(done) {
  new Promise(function(resolve, reject) {
    var a = {};
    a.b = a;
    resolve(a);
  }).then(done).catch(done);
});

Produces

  1) Mocha is confused

  0 passing (20ms)
  1 failing

  1)  Mocha is confused:
     TypeError: Converting circular structure to JSON
      at Object.stringify (native)

The problem here is that there isn't a stack-trace, so I start off with absolutely no idea how to solve the problem. I have no idea why Mocha wants to stringify what I give to the done function, but it does...