> No, what I'm saying is "if adding debug code alters all the bugs, how 
> does that not make it scientifically impossible to remove all the bugs?"

It's virtually impossible to remove them all anyway (heck, you won't 
even /notice/ some of the bugs).

And of course one develops an instinct where such Heisenbugs /really/ 
originate.

It's also the type of bugs where doing a long weekend (or sometimes a 
long week) of "extreme debugging" will usually get you somewhere.

The worse problems are those where the error mechanism is plain obvious, 
but is inherent in the software design, and fixing it invariably creates 
a bug somewhere else, which can only be fixed by introducing a third 
one, which in turn can only be fixed by re-introducing the first one... 
seen such things happening; with such bugs "oscillating" for a month or 
so before people decided to get back to the drawing board, and redesign 
that part of the application. That was an embedded system - in a large 
software project for an insurance company I was briefly involved in, 
some bugs had allegedly been oscillating between "fixed" and "broken" 
for years already.

Fortunately that kind of bugs doesn't suit my working mode: Repetitive 
tasks - i.e. anything I encounter more than once - typically prompt me 
to invest a lot of time and energy into making sure I never have to 
spend any significant time on them a third or even fourth time.

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