On floating points issue and his analysis why it happened.
But wanted to show here that doing the same thing using a CAS removed the problem (as would be expected, nothing surprising, since using pure integers). Ofcourse one can argue about speed and performance in real large numerical problems and all of this.
But I just thought some here might like to see it.
It is finding the Determinant of 2 by 2 matrix of integers. The correct answer is 1.
I tried the example shown on some software I have, here is the result. I run in on matlab 2112a,matlab 2112a/symbolic,octave, Mathematica,Maple: