The fix is in, but is the bug fixed? If you are a programmer you say, “of course.” But how do you know it is fixed? That the bug will be corrected? Certainly, QA will remind you how often “fixed” bugs turned to be nothing but.
Of course, formal proofs are great –but only done in high castles of academia, and perhaps, in the serious business of space, weaponry, and medical systems (or so we hope).
George Neville-Neil Kode Vicious (August 2013 ACM) solution is quite a good one: use the scientific method. That is, write a hypothesis, implement it in the code fix, and explain how it is tested. Or at least, formalize your thinking.