How to verify multiple method calls with different params

If the order of both add() calls is relevant, you can use InOrder:

InOrder inOrder = inOrder(errors);
inOrder.verify(errors).add(eq("exception.message"), any(ActionError.class));
inOrder.verify(errors).add(eq("exception.detail"), any(ActionError.class));

Further reading has led me to try using ArgumentCaptors and the following works, although much more verbose than I would like.

ArgumentCaptor<String> argument = ArgumentCaptor.forClass(String.class);

verify(errors, atLeastOnce()).add(argument.capture(), any(ActionMessage.class));

List<String> values = argument.getAllValues();

assertTrue(values.contains("exception.message"));
assertTrue(values.contains("exception.detail"));