From time to time you have to debug time critical code. If you are using interrupts, you are interested in the actual performance on the code. This is especially true, if you are using timed interrupts.
In this cases, using the serial interface is impossible. If you write to the serial interface, the whole timing of your application changes and you will get no usable results.
If you own an oscilloscope, there is a really simple way to debug many applications. I used this technique many times, for example with projects featured on these pages: For the timing of the sound output of the cat protector, or the timing of the display interrupt of the Meggy Jr library.
This technique is obvious ans simple. Nevertheless I hope this article will give some inspiration to help solving timing problems.