Adding a `case` after the default one in a `switch` control structure still works. Compiled on a POSIX system with `gcc` and flags `-Wall -Wextra -pedantic -std=c89`.

gistfile1.c

#include <unistd.h>

void    say(int x)
{
    switch(x)
    {
        case 0:
            write(1, "Case 0\n", 7);
            break;
        default:
            write(1, "Case default\n", 13);
            break;
        case 1:
            write(1, "Case 1\n", 7);
            break;
    }
}

int     main(void)
{
    say(0); /* Case 0 */
    say(1); /* Case 1 */
    say(-1); /* Case default */
    return (0);
}