After you delete x(3) then what was at x(4) "falls down" to occupy x(3), what was at x(5) "falls down" to x(4), and so on, leaving x one element shorter. You then go to the next "i" element, 4, examining what is now in x(4) which was in x(5)... and you have skipped examining what was in x(4) and is now in x(3).
Eventually you get to the point where you have deleted the last entry in x, making x two elements shorter, but your "for" loop over length(x) is going to be based on the original length of x, so you are going to end up attempting to access after the last element of what x has become.
Lesson: if you delete elements of a vector using a forward-going loop, you need to take into account that the remainder of the vector moves down to fill up the hole. You can code that in, or you can recode to avoid using a loop to delete elements, or you can try your hand at using a loop that does not go forward.
