When Metaprograms Attack!
One of the most cunning, or, to be honest, most stupid, bits of code I ever wrote was a Tcl program that wrote itself. It was a browser for a distributed object directory that would let the user examine and invoke objects across the network.
The browser queried the directory service, used CORBA's reflection mechanism to interrogate objects and wrote new classes to represent the remote objects that it found and the GUI components to display objects, get and set their attributes, invoke methods and display returned values. The entire program was two or three screenfuls of code, but wrote itself to be much larger as it ran.
Unfortunately it contained a bug that occurred infrequently and seemingly at random. I couldn't work out where the bug was in the code that the program wrote because I couldn't put breakpoints into code that didn't exist until runtime. That meant that I couldn't find the real bug, the bug in the code that wrote the code that contained the bug that actually occurred in the running program.
Eventually I rewrote the program in a more traditional style. What a shame. It was so very nearly the coolest code I ever wrote.