20-odd years ago, the college I attended (and worked for) was outfitting one of the computer labs with new network cards; migrating from Token Ring to Ethernet as I recall. To do this, 3COM cards had been bought by the boxful.

We replaced all of the cards, and booted the lab. 1 machine got an IP and then it all went to hell. Every single machine showed up with the same MAC. Think we had to rip and replace to fix it.