Chai as Promised and Chai Things didn't work nicely together.

chai-as-promised-and-chai-things.coffee

#!/usr/bin/env mocha --compilers coffee:coffee-script --reporter spec
(require "mocha-as-promised")()
chai = require "chai"
Q = require "q"
chai.should()
chai.use require "chai-as-promised"
chai.use require "chai-things"

array = [{ a: 1 }, { b: 2 }]
arrayPromise = Q.resolve(array)

# include these tests to see the expected error messages
includeFailingTests = off

describe "The array", ->
  it "should be an array", ->
    array.should.be.an("array") # (1)
  it "should contain something deep equal to { a: 1 }", ->
    array.should.contain.something.that.deep.equals({ a: 1 }) # (2)
  if includeFailingTests
    it "should contain something deep equal to { a: 2 }", ->
      array.should.contain.something.that.deep.equals({ a: 2 }) # (3)

describe "The array promise", ->
  it "should eventually be an array", ->
    arrayPromise.should.eventually.be.an("array") # (4)
  it "should eventually contain something deep equal to { a: 1 }", ->
    arrayPromise.should.eventually.contain.something.that.deep.equals({ a: 1 }) # (5)
  if includeFailingTests
    it "should eventually contain something deep equal to { a: 2 }", ->
      arrayPromise.should.eventually.contain.something.that.deep.equals({ a: 2 }) # (6)

The old version in which this problem occurred, has been updated to use a flag instead of a customized Assertion object, resolving the problem.