Because you're removing elements as you go, your array gets shorter, and your fixed index (1:7) can go out of bounds.
This is exactly what is happening when you have:
x1=7, xplus=[5;6;7;8;0]; xplus(x1)=[];
You're trying to remove the 7th element, when the array only has 5 elements in it.
A better way of accomplishing what you're doing is with logical indexing;
xplus(xplus < 0 | xplus > 8) = [];
which finds the elements of x < 0 or > 8 and removes them.
