Worked for a small ISP in early 2000s. Not every new customer had an ethernet card in their PC and we had problems like installation crews calling the office that there is no port to stick the cable in.

The company obtained 300 cheap Intel PRO/100 Fast Ethernet NICs from China. The idea was that the crews would have a bunch of those cards to be able to install them in customers' computers.

Everything was dandy until second installation in the same building. For a little more backstory - it was a kind of shitshow infra with dumb switches and all customers in a building being in the same LAN.

So... the second installation in the same building rendered the former newest customer's link inoperable. Or wouldn't work, then suddenly there was connectivity, then there was no ping to the gateway but some other devices were pingable etc etc. The crew replaced the card with another one (from the same shipping), re-crimped RJ45s on both ends, changed switch port and nothing - same shit with random internet connectivity losses. Weird situation with seemingly no reason for what was happening.

It later turned out that the reason was that all 300 NICs had the same MAC address. All of them.