Mark Chu-Carroll explains the backdoor entry that Ken Thompson included in the C compiler (via Jason Kottke). This backdoor entry is mostly invisible. Why? Because we always consider the source code to be the only source for the compiled output. This hack uses the compiler that compiles it!
The hack is really invisible because it generates the code that inserts the backdoor entry. Being a compiler, it can make sure that binaries compiled with it, including itself, carry forward the hack. This is the power that code generators have, and with metaprogramming, this can really become mainstream. That is, not inserting backdoor entries, but the concept of carrying forward enforcements.