This means you have a line of code somewhere that is asking for item Y out
of a group of items that only has X items. Start up the project in debug and
set breakpoints. Step through until you hit the error. It will be a line
similar to this:

object o = groupofObjects(y);

Find out how big the array is and you will see the issue.

A common reason is start with object #1 as 1 instead of 0 (ordinal versus
cardinal basis). When you try to grab object #10 as 10, it blows up.