Just living far from major datacenters is enough. I get truncated downloads pretty regularly, maybe a couple times a month or so. The network isn't really all that reliable when you consistently use it across the globe.

It usually happens on large files though, due to simple statistics, but given enough users, not hard to imagine it happening with a small script...

That's easily fixed by adding Content-Length headers.

You pull the Ethernet cable out before it finishes. Or your wifi router hiccups

Wouldn’t the download terminate without emitting the script?