I'm using a modified Parallax theme, copied over from a different shop that I also run.
For some reason, the product photos fail to render about 25% of the time when you load the product page. It worked perfectly fine on the original store that I copied the theme from, and the issue is only present on this store. I did make some minor cosmetic modifications, but none of it should be related as far as I am aware.
I think it has something to do with Flickity not initializing properly, as none of the Flickity HTML classes are present when it fails to load the images.