Be doubly-certain that the incomming CDC class driver's endpoint/pipe is flushed only once when the bank is empty.

LUFA/Drivers/USB/Class/Device/CDC.c

@@ -190,10 +190,17 @@ uint16_t CDC_Device_BytesReceived(USB_ClassInfo_CDC_Device_t* const CDCInterface
 
 	Endpoint_SelectEndpoint(CDCInterfaceInfo->Config.DataOUTEndpointNumber);
 
-	if (Endpoint_IsOUTReceived() && !(Endpoint_BytesInEndpoint()))
-	  Endpoint_ClearOUT();
+	if (Endpoint_IsOUTReceived())
+	{
+		if (!(Endpoint_BytesInEndpoint()))
+		  Endpoint_ClearOUT();
 
-	return Endpoint_BytesInEndpoint();
+		return Endpoint_BytesInEndpoint();
+	}
+	else
+	{
+		return 0;
+	}
 }
 
 uint8_t CDC_Device_ReceiveByte(USB_ClassInfo_CDC_Device_t* CDCInterfaceInfo)

LUFA/Drivers/USB/Class/Host/CDC.c

@@ -309,13 +309,22 @@ uint16_t CDC_Host_BytesReceived(USB_ClassInfo_CDC_Host_t* const CDCInterfaceInfo
 	Pipe_SetPipeToken(PIPE_TOKEN_IN);
 	Pipe_Unfreeze();
 
-	if (Pipe_IsINReceived() && !(Pipe_BytesInPipe()))
-	  Pipe_ClearIN();
-	
-	BytesInPipe = Pipe_BytesInPipe();
-	Pipe_Freeze();
-	
-	return BytesInPipe;
+	if (Pipe_IsINReceived())
+	{
+		if (!(Pipe_BytesInPipe()))
+		  Pipe_ClearIN();
+		
+		BytesInPipe = Pipe_BytesInPipe();
+		Pipe_Freeze();
+		
+		return BytesInPipe;
+	}
+	else
+	{
+		Pipe_Freeze();
+		
+		return 0;
+	}
 }
 
 uint8_t CDC_Host_ReceiveByte(USB_ClassInfo_CDC_Host_t* const CDCInterfaceInfo)