Address review comments.

csharp/src/Google.Protobuf.Test/GeneratedMessageTest.cs

@@ -629,5 +629,27 @@ namespace Google.Protobuf
             var data = new byte[] { 130, 3, 1 };            
             Assert.Throws<InvalidProtocolBufferException>(() => TestAllTypes.Parser.ParseFrom(data));
         }
+
+        /// <summary>
+        /// Demonstrates current behaviour with an extraneous end group tag - see issue 688
+        /// for details; we may want to change this.
+        /// </summary>
+        [Test]
+        public void ExtraEndGroupSkipped()
+        {
+            var message = SampleMessages.CreateFullTestAllTypes();
+            var stream = new MemoryStream();
+            var output = new CodedOutputStream(stream);
+
+            output.WriteTag(100, WireFormat.WireType.EndGroup);
+            output.WriteTag(TestAllTypes.SingleFixed32FieldNumber, WireFormat.WireType.Fixed32);
+            output.WriteFixed32(123);
+
+            output.Flush();
+
+            stream.Position = 0;
+            var parsed = TestAllTypes.Parser.ParseFrom(stream);
+            Assert.AreEqual(new TestAllTypes { SingleFixed32 = 123 }, parsed);
+        }
     }
 }

csharp/src/Google.Protobuf/CodedInputStream.cs

@@ -346,7 +346,7 @@ namespace Google.Protobuf
             switch (WireFormat.GetTagWireType(lastTag))
             {
                 case WireFormat.WireType.StartGroup:
-                    ConsumeGroup();
+                    SkipGroup();
                     break;
                 case WireFormat.WireType.EndGroup:
                     // Just ignore; there's no data following the tag.
@@ -367,7 +367,7 @@ namespace Google.Protobuf
             }
         }
 
-        private void ConsumeGroup()
+        private void SkipGroup()
         {
             // Note: Currently we expect this to be the way that groups are read. We could put the recursion
             // depth changes into the ReadTag method instead, potentially...