Does anyone know why my variable (x) changed its value from 3 to 0 (1 after x++) in this program? I'm trying to make a signup system but it overwrites

Question

Like what the title said, I was trying to make a signup system but for some reason it overwrites the array which was supposed to be defaultuser[0] = "12291955" it becomes what I inputted except I wanted it to go to defaultuser[3] (denoted by x = 3). So basically, the signup system is supposed to add a 4th user. (The following is a header file for my main.c file)

#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
#include <string.h>

void loginsystem(){
    char defaultuser[][100] = {"12291995", "12221999", "12241344"};
    char defaultpass[][100] = {"test1", "test2", "test3"};
  

    int i = 0, indicator = 1, x = 3;
    char userinput[20], passinput[20], choicest[20], prog[20]; 
    const char* choicecmp;
    system("cls");
    printf("\nSystem");
    printf("\nWould you like to create an account or log into an existing account? (Type Signup or Login) ");
    scanf(" %[^\n]s", choicest);
    printf("%d", x); //x=3
    choicecmp = strlwr(choicest);
    if(strcmp(choicecmp, "signup")==0 || strcmp(choicecmp, "sign up") == 0 )
    {
        //system("cls"); 
        printf("\nEnter your ID Number: ");
        scanf(" %s", defaultuser[x]);
        printf("\nEnter your password: ");
        scanf(" %s", defaultpass[x]);
   
        getch();
        choicecmp = "login";
        x++; 
        printf("%d", x); //x becomes 1 for some reason
        getch();
    }
    if(strcmp(choicecmp, "login")==0 || strcmp(choicecmp, "log in") == 0 ){
    do{
    system("cls");
    printf("\nPlease login!");
    printf("\n\nUsername (ID Number): ");
    scanf(" %s", &userinput);
    printf("\nPassword: ");
    do{
            passinput[i] = getch();
            if(passinput[i] != '\r')
            printf("*");
            i++;
        }while(passinput[i-1] != '\r');
        passinput[i-1] = '\0';

    for(int i = 0; i < x; i++){ 
    if(strcmp(userinput, defaultuser[i]) == 0 && strcmp(passinput, defaultpass[i]) == 0)
    {
        system("cls");
        indicator = 0;
        printf("\nWelcome back User");
    }}
    return ;
}

I tried debugging the code and turns out that int x = 3 became x=0 along the way in the signup system, and became 1 after the x++.

Answer 1

defaultuser has three elements.

char defaultuser[][100] = {"12291995", "12221999", "12241344"};

But you try to write to the fourth element.

int x = 3;
scanf(" %s", defaultuser[x]);

This is undefined behaviour.