What is a bug? If it's not what the programmer wanted, it's a bug. The machine only executes what it is asked to execute, it doesn't know if it is what the programmer really wanted to do. It's the same problem with perfectly legal tax optimizations, it is legal but when it is abused to circumvent things, is it what a civilization wanted? The rules define how things happen, a bug/exploit will always be a valid mechanic in a sense where the program only did his job following the rules.