http://www.w3.org/TR/WD-frames-970331Infinite Recursion
It is possible to design a FRAMESET that recurses infinitely, stacking the same FRAMESET inside itself until a machine runs out of memory. Consider this example:
<FRAMESET rows="50%,50%">
<FRAME src="parent_url">
<FRAME>
</FRAMESET>
Infinite recursion is prevented. Any frame that attempts to assign as its SRC a URL used by any of its ancestors is treated as if it has no SRC URL at all (basically a blank frame). This doesn't prevent all malicious documents, but it eliminates a troublesome class of them.