It is not true that free software all just ships and is tested by users. There are many segments of the community which have strong testing philosophies. For instance any decent CPAN module comes with regression tests.

However Linux distributions inevitably throw out all of these tests. I know that my Ubuntu distribution has problems with the locale. When I install CPAN modules by hand, their tests break on that. But the same CPAN modules are available from within Ubuntu, and I have every reason to believe that they still would not pass their unit tests. But there is no way that someone using the Linux distribution would notice that fact. And it is far from obvious where someone like me should report the bug.

Cheers,
Ben