On testing.

Glenford Myers - "The Art of Software Testing"

Very good book.

Every paragraph could be cited.

Example cite:

"
In an ideal world, we would want to test every possible permuta-
tion of a program. In most cases, however, this simply is not possible.
Even a seemingly simple program can have hundreds or thousands of
possible input and output combinations. Creating test cases for all of
these possibilities is impractical. Complete testing of a complex appli-
cation would take too long and require too many human resources to
be economically feasible."

From my experience, the design of a program could be also so complicated and not straightforward, that even coverage of all possible control flows with existing resources will be practically impossible.

And that way, there are no other way rather than redesign this program in "testable" way (by restructuring and simplifying internal structure).