At a very former workplace, we suddenly lost the ability to ARP between two buildings. Once the ARP entry was in the machines, it worked fine (FSVO "fine"), but getting to the point where ARP worked simply was not reliable.
Much troubleshooting later, it turns out that when they'd been doing some maintenance in the lift shaft (which was also used to drop the inter-building 10Base-5 yellow snake), they'd managed to shoot a nail through the Ethernet cable and we now had a nice 50Hz hum on the cable.
Retries in TCP made that work, but ARP doesn't have retries, so if that managed to get faded out, you'd hope to get lucky next time...
Much troubleshooting later, it turns out that when they'd been doing some maintenance in the lift shaft (which was also used to drop the inter-building 10Base-5 yellow snake), they'd managed to shoot a nail through the Ethernet cable and we now had a nice 50Hz hum on the cable.
Retries in TCP made that work, but ARP doesn't have retries, so if that managed to get faded out, you'd hope to get lucky next time...