My favorite kind of killer bug is the Heisenbug - a bug that changes its behavior depending on how you observe it. In the general case, it goes away as soon as

My favorite kind of killer bug is the Heisenbug - a bug that changes its behavior depending on how you observe it. In the general case, it goes away as soon as you start to use a debugger or put logging statements into your code. The most common bug of this sort involves race conditions and subtle corner cases wrt timing, in systems with independent asynchronous processes interacting in ways that are difficult or impossible to model using formal methods.