Sorry to drag this thread back up, but it is now the #1 hit for the UnexpectedCallError in the OP. Just to note that beyond the more esoteric situations discussed in the thread so far, you can also see this error via a simple race condition. This situation is discussed in the doco under Blocking on Expectations at Mox — Mox v1.2.0. It also comes up a few times in the closed Mox issues. Basically if the test finishes before any spawned processes then Mox will tear down the expectations before the tested behaviour uses it. Can be very confusing as you may partial mock hits where it is set to expect multiple hits. One way around it is to force the test to wait by messaging from within the mock. From that doco:
test "calling a mock from a different process" do
parent = self()
ref = make_ref()
expect(MyApp.MockWeatherAPI, :temp, fn _loc ->
send(parent, {ref, :temp})
{:ok, 30}
end)
spawn(fn -> MyApp.HumanizedWeather.temp({50.06, 19.94}) end)
assert_receive {^ref, :temp}
verify!()
end
Hope that helps someone in a similar situation!