Attempting to insert into a shelf being used in a repeat over, an item which is less than or equal to the size of the shelf at the start of the repeat over.

#6039 Run-time catchable error

For STREAM/COUNTER/SWITCH 'X', BEFORE/AFTER item Y, repeat over size of Z.

It is not allowed to insert items to a shelf subject to a repeat over unless the inserted item is added at the right end of the shelf.