There's a major exception that needs to be stated here. If you're writing anything with security implications -- anything that handles users' valuable personal

There's a major exception that needs to be stated here. If you're writing anything with security implications -- anything that handles users' valuable personal data, particularly financial -- you had damn well better know what you're doing and think about it carefully. If your site gets hacked and credit cards get stolen, your users will have a crappy experience, no matter how spiffy your site is otherwise.