Being Defensive About Defensive Programming
Too much defensive programming creates problems of its own. If you check data passed as parameters in every conceivable way in every conceivable place, your program will be fat and slow. What's worse, the additional code needed for defensive programming adds complexity to the software. Code installed for defensive programming is not immune to defects, and you're just as likely to find a defect in defensive-programming code as in any other code—more likely, if you write the code casually. Think about where you need to be defensive, and set your defensive-programming priorities accordingly.
Too much of anything is bad, but too much whiskey is just enough.