I remember buying a bunch of old HP ISA 100Mbit NICs to turn an old computer into a router/server combo. Naive as I was I put them all in and nothing worked. Turns out they were all configured to use the same IRQ (since they likely came from independent machines), and that caused them to overwrite each others settings… including the MAC adress. Thankfully I found some nice hacker that worked with these cards before and published a little C tool to rewrite their EEPROMs. I contacted him if he sees a chance to resurrect the cards and that saint indeed hacked the necessary features into his tool so I could rewrite the MAC adresses, change the IRQ one by one and ended up with a working network. Good times.