In the early days of network interface, unique IDs were a problem. It was once suggested that each network interface have a $1 bill attached, with the serial number of the bill being the adapter ID.

This was a real problem in the early days of low-cost Ethernet controllers. Some manufacturers didn't buy their own address space [1], but reused that of some major vendor. (Usually 3COM) This resulted in occasional real-world duplicates.

[1] https://regauth.standards.ieee.org/standards-ra-web/pub/view...