Look at these solutions. [link1][1] [link2][2] [link3][3]
All these gives an answer 1 for input 1 which should never happen(because 1 is not a prime number). But they are given AC. This is due to weak test cases.
hoping that @admin would not just leave it aside and update the test cases.
On observing, most of the brute force solutions fall into this category.
[1]: http://www.codechef.com/viewsolution/3283760
[2]: http://www.codechef.com/viewsolution/3280288
[3]: http://www.codechef.com/viewsolution/3282969