I was on this page:
http://sf.eater.com/maps/best-new-san-francisco-restaurants-heatmap-oakland-berkeley

and I tried the new download menu item in Clank. It spun for quite a while (I was on fast WiFi) and then indicated "Download failed".

If I do it again, it succeeds immediately. If I go away and come back such that the page is reloaded and download again, that fails, then succeeds when I do it again. I tried a few other random pages and they downloaded OK. It seems semi-reproducible on this page.

This reproduced in dev (54.0.2840.16) and canary (55.0.2853.3)