Here is an example that one of the best "rocks to look under" (as in pg's "What You Can't Say" essay) is the misapplication of fundamental laws:

http://news.ycombinator.com/item?id=2254906

The researchers who wrote this paper looked under that rock. (In this case, pop-science application of fundamental physical law. It's hard to get more fundamental than thermodynamics.)

(An example for you entrepreneurs out there, I'd suggest looking under Kerchoff's Law. Yes, it's a great principle to keep in mind when designing security tools like ciphers and cryptographic protocols. However, I contend it's applied in a counter-productive way when people think about real-world systems -- those have boundaries that are way too complicated for the application of Kerchoff's Law. Often, this is just an opportunity for someone to show they understand crypto theory, then throw up their arms and declare nothing can be done. However, the truth is that security often works or doesn't work because of the economics!)