create procedure get_C (inout b any)
{
  return cast (xpath_eval('//C', b) as varchar);
}

create procedure hidden_bug_1()
{
  declare abc any;      -- some XML document
  declare b_list any;   -- a vector of all B elements of the doc
  declare c1 varchar;   -- a name of first variable
  declare c2 varchar;   -- a title of second variable
  abc := xtree_doc ('<A><B
xml:id="1_01"><C>One</C></B><B
xml:id="2_01"><C>Two</C></B></A>');
  b_list := xpath_eval ('//B', abc, 0);
  -- now b_list is a vector of two items, '<B
xml:id="1_02"><C>One</C></B>' and '<B
xml:id="2_02"><C>Two</C></B>'.
  c1 := get_C ( aref (b_list, 0));
  -- looks fine, c1 is '<C>One</C>'
  c2 := get_C ( aref (b_list, 1));
  -- looks strange fine, c2 is '<C>One</C>',
  -- while the expected value is '<C>Two</C>'.
}
      

The origin of the bug is '//C' path in get_C(), which returns not the "C element inside given b element", but "C element inside the document where given b element is located", thus get_C returns the first C element in the whole document with any of two B elements given.

There are two ways to fix this bug. It is better to correct get_C():

create procedure get_C (inout b any)
{
  return cast (xpath_eval('./C', b) as varchar);
}
      

If you cannot patch get_C() for some reason, xml_cut will help, but it will waste both memory and CPU time for copying a branch of XML tree:

create procedure hidden_bug_1()
{
...
  c1 := get_C ( xml_cut (aref (b_list, 0)));
...
}