Wax and clay seals have always struck me as a rather imperfect security mechanism. Better than nothing, of course, but imperfect. So I was interested to learn that Los Alamos National Laboratory's Vulnerability Assessment Team had conducted a study, "Were ancient seals secure?"
Abstract: Forgeries of ancient seals have been found in modern times, but there has been little previous analysis of how much security ancient seals might have offered. In this paper, we demonstrate four different vulnerabilities of clay seal impressions using attack methods and materials that were available thousands of years ago. The success of these attacks suggests that ancient stamp and cylinder seals may have been highly vulnerable to spoofing.
However, the authors don't mention any known instances of ancient seal-spoofing, let alone any consequential instances of spoofing. But the authors are not historians, so I don't take this as indication that it never happened.
So, did a forged seal ever seriously compromise a political or military operation?
A forged seal could be effective in at least two ways. First, you could send out counterfeit orders, telling a vassal to march west when the king or whoever really wants the vassal to march east. Second, you could conceal an act of espionage by resealing an order: the vassal receives the order to march east, and because the seal is intact the vassal does not take extra precautions, only to be ambushed along his route. You can imagine plenty of other scenarios.
Given the several thousand years of history when seals were an important security mechanism, I imagine that a forged seal was pivotal at one point--but I haven't found any examples yet.