>It is sufficient to prove a program correct - as long as your proof is not faulty!

This is unquestionable truth. Proof:

Proposition A(X): X is true in theory

Proposition B : For all X such that A(X), X is true in practice

Theoretically, there is no difference between theory and practice. ... (1)

Theoretically, statement B is true. [using (1)] ... (2)

Therefore, B is true in practice. [using (1) and (2)]

QED.