You have a variable defined:

a_type my_var[5][5];

Then you iterate:

for (i = 0; i < 6; i++) {
    for (j = 0; j < 6; j++) {
    }
}

Now a question: If your array has valid index values [0, 1, 2, 3, 4] (in total 5 values), what happens if you copy your array using a for loop taking the values [0, 1, 2, 3, 4, 5] (in total 6 values)?

Hint: Define memory overwrite.
Hint: Define buffer overflow.
Hint: Define index-out-of-range.