Undefined behaviour springs to mind. A statement invoking UB is neither syntactically nor semantically incorrect, but rather the result of the code cannot be predicted and is considered erroneous.
An example of this would be (from the Wikipedia page) an attempt to modify a string-constant:
char * str = "Hello world!";
str[0] = 'h'; // undefined-behaviour here
Not all UB-statements are so easily identified though. Consider for example the possibility of signed-integer overflow in this case, if the user enters a number that is too big:
// get number from user
char input[100];
fgets(input, sizeof input, stdin);
int number = strtol(input, NULL, 10);
// print its square: possible integer-overflow if number * number > INT_MAX
printf("%i^2 = %i\n", number, number * number);
Here there may not necessarily be signed-integer overflow. And it is impossible to detect it at compile- or link-time since it involves user-input.