Yes, it’s the engineer’s fault. OTOH, it’s QA’s fault for not catching that mistake, and the company’s fault for releasing a product that wasn’t properly tested.

We’re talking about Cisco here, a company that sells millions of units. The more units you expect to sell, the more extensive your QA procedures need to be. It’s not like this is their first piece of networking gear either. Maybe they’ve never had this specific error before, but surely they’ve had errors caused by people using a variety of different kinds of ethernet cables. I would imagine they have tests where they plug a dozen different kinds of ethernet cables into every new product they make just to ensure that a cable that has given them problems before doesn’t have issues with this new piece of gear.

When a problem like this is caught by QA people, it’s mostly the engineer’s fault for a design mistake. When these errors are caught in the wild by customers, it’s the company’s fault for a screw-up somewhere in their QA / test / release procedures.