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Oh, that's quite funny, because it looks like HttpMethod stores everything that have been requested:

    @Test
    void weirdVerbs() {
        Unirest.request("CHEESE", MockServer.CHEESE)
                .asObject(RequestCapture.class)
                .getBody()
                .assertMethod(HttpMethod.valueOf("CHEESE"))
                .assertBody("");
    }

Results later in:

Error:    
HttpMethodTest.shouldReturnAllMethods:51 
expected: <[PATCH, GET, OPTIONS, DELETE, POST, PUT, TRACE, HEAD]>
  but was: <[PATCH, CHEESE, GET, OPTIONS, DELETE, POST, PUT, TRACE, HEAD]>

It looks like custom methods are considered as a feature, so what do you think about extending HttpMethod with a property canonic/standard, so we can differ those methods? Or we can just remove this test if the registry it meant to keep non-standard values for the whole jvm instance.

I'd just change the test to:

assertEquals(HttpMethod.GET.all(), HttpMethod.all());

You could also check that it contains the default (but might have more which is fine).

but yes, there re some systems with custom methods. mostly old