Turns out Heisenberg was right for computer programs as well.
Watch out for heisenbugs, and before you try to fix a bug; toggle the state of your log system.
If a bug changes behavior when adding or removing printouts, check you timing constraints.
This leads to two conclusions:
a) You need a log system that is easy to turn on and off
b) Off means off, preferably unused code should not be compiled/called
PS: Adding a printout in order to fix a bug is not a fix ;)