How to create C program that is proof for any random test cases?

Sometimes I enroll myself in coding competition held by various IT companies. Many of the time I had successful to write a program according to their requirements. I test them with the given examples and my own input and always get the desired result but when I upload them the automatic compiler gives me an Runtime error failure.

I think they test the program with random test cases in which my program fails. How to write a C program or what are the steps I should keep in mind while writing a C programming code that will survive any arbitrary test cases?

Thanks in advance.