It turned out to be something different, but another much more common source of shader output differences is when a float value is clamped to integer (with the value being very close to the closest integer), and then use the clamped value as index (or tex coord with an unfiltered sampler). This may result in an off-by-one error on some gpu/driver combos but not others. Completely understandable why it happens, but hard to catch unless testing on a wide range of GPUs and drivers.

TL;DR: don't expect that floating point operations on GPUs are strictly IEEE-754 compatible