Imagine that your website has been hacked and defaced. Owning and running a good website can be challenging. Having your website hacked can be a nightmare and dealing with a hacked website, an even scarier one.
Hack
When things don't work the way you want it to and all it takes is a hack in the core, people sometimes hack the core. You might get the feature working the way you wanted it to but by doing that you would effectively be crippling your drupal installation. There are three main reasons why you should not hack core in Drupal.