This is called the Heisenberg Principle. It origins from physics (see the wikipedia entry: http://en.wikipedia.org/wiki/Uncertainty_principle). The idea is that when want to measure a system, you insert a probe to measure it, and thus you change the system. It applies to software as well (read the Popular culture part of the wikipedia entry for more "implementations" of this principle).

The application crashes due to a race condition, you put a
Qt Code: 
  1. printf()
To copy to clipboard, switch view to plain text mode 
which changes the internal timings (or compile in debug mode, which might be slower, faster, or just different), different commands get scheduled differently - oops, no problem

Now the fun part of my responce: good luck, you will need it.