Best one was a Netware 3.12 customer, their server had two network cards in it to support the two network runs around the office on Thin Net, each about 150m ish in length.
Get a support call one day that the clients keep dropping so I grabbed our network kit (BNC tester, 2 spare NE2000 cards, 3 spare terminators, a few T's and a couple of 5M lengths of thin net) and head out to the customers site.
Upon arriving I notice that the server has moved to the other side of the office, and both segments are connected to ONE card.
I ask why this is the case and they admitted to losing one of the terminators when moving the server and of course they did not see the issue, I measured the network before I fixed it and sure enough, total length 327M :)
Re-terminated the second card, split the segments and of course the network issues disappear immediately.
Easiest emergency callout fee we ever earned.