For Safari users seeing persistently confusing or inconsistent results: it’s also worth noting that Safari aggressively caches and ignores cache-invalidating HTTP headers quite a lot. So you might just be seeing what’s in memory, over and over.
Er, the different experiences Safari users are reporting may very well have to do with how it’s showing cached resources. If some people are reporting frequently different results for a race condition and others are reporting stable results, aggressive caching very well might be a factor.
When your browser caches an image it doesn't cache the pixel buffer itself, it just cached the original file (the PNG in this case). Because the PNG is non-deterministic it can render differently every time you open it even if you save it to disk and open it from there.
It's a race condition. Under the right circumstances, eg hardware at the right speed, under the right load, it might very well only result in one thing or another. The page is only ever serving one image anyways, so you can cache it as hard as you want.
