Comment by NateLawson

One of my fellow ISP admins (silicon.net, I was elite.net) had the same problem around 1995. GIFs would load on web pages but not JPEGs. They'd load for some amount of time and then hang.

The way he diagnosed it was to do a transfer of /dev/zero out one link. It worked. But it stopped almost immediately out the problem link. It turned out to be the same problem -- no zero bit stuffing configured on the line.

By the way, this same technique, known as "weak bits", was used as a floppy and CD/DVD copy protection scheme.