• Reliability does not imply safety. For example, if there were only a one in a million chance per hour that a program would do the wrong thing, but that wrong thing would kill a million people, most would agree that the software is "unsafe". It is notoriously difficult to quantify the probability of a piece of software doing the wrong thing, since such quantification usually leaves out important possibilities, so even when someone says "only one in a million" the actual probabilities are usually much greater.
• "Extensive testing" is always an illusion - it's impossible to test most real programs for all possible circumstances, so testing only handles a very small subset of the actual situations the program will encounter. Trivial programs that have only ten 16-bit integers have 2^160 different possible states; such a trivial program couldn't be totally tested in the lifetime of the universe.

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