Or both systems are wrong, but just wrong in different ways.

Or both are right, and the software is hitting some undefined behaviour in the compiler. The article does in fact mention that a different compiler version was used.

With C is quite easy to happen, don't know about fortran though.

Precisely. My point was don't assume one system is right, the other wrong.

Touched upon compiler (internals and flags) in a separate post.

I would call an error placed by the compiler due to failure to exactly follow standards a problem in the system.

We are talking totality of systems here, the compiler that produces the programs is part of the system that uses the program.

I wasn't referring to failures of the compiler to follow the standards, but rather to failures of the _software_ to follow the standard.

"So assuming the two programs running are in fact the exact same,"

The source code for the programs might as well be the same, but the actual (binary) program is not necessarily the same.

Or to put it in other words, both systems might be perfectly correct, but there is a class of software bugs that get revealed only when incarnated after being compiled in one of the several perfectly legit ways. The dimension of this set of possible alternative optimisations can be quite high if your language offers many areas for undefined behaviour.

Does anybody know if fortran is plagued by it as much as C ?

Not necessarily. The Fortran standard allows for floating-point reordering that can produce value errors. All that's required is that the new system's compiler optimize differently than the last one; there isn't necessarily a bug anywhere. See https://software.intel.com/sites/default/files/article/16438...