https://github.com/angular/angular.js/blob/master/src/service/xhr.js#L154

When executing callback (after xhr request), all exceptions are caught and just reported into $log.error().
That's not enough for testing !

Imagine this:
I'm testing controller / service or whatever. And there is xhr request and then, any exception in the callback !
Then the test passes, you have to explicitely check the $log service....

expect(scope.$service('$log).error.logs.length).toBe(0);