No Algorithm Can Survive First Contact With Real World Data

Your algorithm passes all unit tests with flying colors. Integration tests? Green across the board. You deploy to production feeling like a genius. Then real users show up with their NULL values in required fields, negative ages, emails like "asdfjkl;", and suddenly your code is doing the programming equivalent of slipping on ice while being attacked by reality itself.

The test environment is a sanitized bubble where data behaves exactly as documented. Production is where someone's last name is literally "DROP TABLE users;--" and their birthdate is somehow in the year 3000. Your carefully crafted edge cases didn't account for the infinite creativity of actual humans entering data.

Fun fact: This is why defensive programming exists. Trust nothing. Validate everything. Assume users are actively trying to break your code, because statistically, they are.