You're using sizeof(s) for the length of the string; it should be strlen(s).

How does this cause the weird behaviour you saw? The value of sizeof(s) will always be 8, the number of bytes used to represent a pointer. So your code is always reading 8 characters, regardless of how long the string really is. If the string is shorter than 8 characters, the code reads some of whatever happens to sit after it in memory - which is probably the string for the next test. So by commenting out some tests, you affect the behaviour of your code on other tests.

As you can see it's not a problem of the kata... Top of the page: 171 guys passed the C kata out of 1991. It's a CW error. You can report it at CW as a bug.