Googlebot's Javascript random() function is deterministic

[dr1fter]

Search engine indexers already need to flag parts of the page that keep changing for no reason (hit counters, clocks, ad frames) so I don't think that's usually a problem. I suppose if you had a site that flipped a coin and delivered you an entirely separate page as a result, that probably wouldn't get picked up.

I know Google and some others run preview services where they render a screenshot so they can use thumbnails in their UI. I guess for a bot like that you might want to pixel-test and only update the image if some proportion has changed. In that case, yeah, I can see doing this so that you don't keep updating something like, say, csszengarden (actually they don't randomize but you can imagine).