Spring Boot currently registers an endpoint with the servlet container to process errors. This means that MockMvc cannot be used to assert the errors. For example, the following will fail:

mockMvc.perform(get("/missing"))
    .andExpect(status().isNotFound())
    .andExpect(content().string(containsString("Whitelabel Error Page")));

with:

java.lang.AssertionError: Response content
Expected: a string containing "Whitelabel Error Page"
     but: was ""
    ..

despite the fact that the message is displayed when the application is actually running. You can view this problem in https://github.com/rwinch/boot-mockmvc-error

It would be nice if Spring Boot could provide hooks to enable testing of the error pages too.