law of unintended consequences
Yes, a.k.a. Chaos theory, Complexity theory, Fog of War, or Friction... But what real makes me cringe (and reach for the soapbox to stand on) is that most of these consquences are fairly forseable. Example: The (thankfully failed) C.I.A. efforts to kill Fidel Castro in the '60s - did they really want Raul Castro (as he was then, he seems to have mellowed a bit) running the show in Cuba? Did they assess the implications of a compromised operation? I'm pretty sure the answer is 'no' to both, but there is a serious issue with the degree of thought applied to the problem.

O.K. I'll move away from the soapbox.

On a more pragmatic note; the assassination of Yamamoto represents a near ideal case. A clean hit with strategicly trivial direct collateral damage, and little if any long term negative impact to U.S. interests... Unless you count the belief that we can repeat this miracle.