There's speculation that Israel may have left a calling card buried in the computer worm believed to be targetting Iran's nuclear programme. From The New York Times:
Deep inside the computer worm that some specialists suspect is aimed at slowing Iran’s race for a nuclear weapon lies what could be a fleeting reference to the Book of Esther, the Old Testament tale in which the Jews pre-empt a Persian plot to destroy them.
That use of the word “Myrtus” — which can be read as an allusion to Esther — to name a file inside the code is one of several murky clues that have emerged as computer experts try to trace the origin and purpose of the rogue Stuxnet program, which seeks out a specific kind of command module for industrial equipment.
Not surprisingly, the Israelis are not saying whether Stuxnet has any
connection to the secretive cyberwar unit it has built inside Israel’s
intelligence service. Nor is the Obama administration, which while talking about
cyberdefenses has also rapidly ramped up a broad covert program, inherited from
the Bush administration, to undermine Iran’s
nuclear program. In interviews in several countries, experts in both
cyberwar and nuclear enrichment technology say the Stuxnet mystery may never be
There are many competing explanations for myrtus, which could simply signify
myrtle, a plant important to many cultures in the region. But some security
experts see the reference as a signature allusion to Esther, a clear warning in
a mounting technological and psychological battle as Israel and its allies try
to breach Tehran’s most heavily guarded project. Others doubt the Israelis were
involved and say the word could have been inserted as deliberate misinformation,
to implicate Israel.