diff --git a/lib/mock-ajax.js b/lib/mock-ajax.js
index 224c13d..7cc57b5 100644
--- a/lib/mock-ajax.js
+++ b/lib/mock-ajax.js
@@ -249,6 +249,9 @@ WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
       responseText: null,
 
       response: function(response) {
+        if (this.readyState === 4) {
+          throw new Error("FakeXMLHttpRequest already completed");
+        }
         this.status = response.status;
         this.statusText = response.statusText || "";
         this.responseText = response.responseText || "";
@@ -261,6 +264,9 @@ WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
       },
 
       responseTimeout: function() {
+        if (this.readyState === 4) {
+          throw new Error("FakeXMLHttpRequest already completed");
+        }
         this.readyState = 4;
         jasmine.clock().tick(30000);
         this.onreadystatechange('timeout');
diff --git a/spec/javascripts/fake-xml-http-request-spec.js b/spec/javascripts/fake-xml-http-request-spec.js
index 197414f..8751a86 100644
--- a/spec/javascripts/fake-xml-http-request-spec.js
+++ b/spec/javascripts/fake-xml-http-request-spec.js
@@ -81,6 +81,22 @@ describe("FakeXMLHttpRequest", function() {
         expect(xhr.readyState).toEqual(4);
         expect(xhr.onreadystatechange).toHaveBeenCalled();
       });
+
+      describe("when a second response comes in", function() {
+        it("should throw an error", function() {
+          xhr.onreadystatechange.calls.reset();
+          xhr.response({status: 200});
+          expect(function() { xhr.response({status: 200}); }).toThrowError('FakeXMLHttpRequest already completed');
+        });
+      });
+
+      describe("when a second response comes in as a timout", function() {
+        it("should throw an error", function() {
+          xhr.onreadystatechange.calls.reset();
+          xhr.response({status: 200});
+          expect(function() { xhr.responseTimeout(); }).toThrowError('FakeXMLHttpRequest already completed');
+        });
+      });
     });
 
     describe("when aborted", function() {