How to reproduce:
Create a block element with position relative. Let's call it grandparent.
Put some text in it so that it has non-zero height
Create a block element inside grandparent with position absolute. Let's
call it parent.
Set parent to have top: 0; bottom: 0; left: 0; right: 0;*
Create a block element inside parent with position absolute. Let's call it
Set child to have top: 0; bottom: 0; width: 1em; border: 1px dotted red;
In Opera 9.5 child has zero height. In all other browsers tested** child
has the same height as parent less top + bottom, which in this case as top
and bottom are 0 means the same height as parent.
*or width: 100%; height: 100%; or probably anything else for that matter
**IE7, IE8, FF2, FF3, Safari, Opera 9.27