Back in my NetWare days
We had a moderate-sized client. 5 UK offices, plus two or three overseas (which luckily didn't factor into this!). IPX across the board, with a bunch of 3Net routers linking the UK offices together. All worked pretty well until one day the phone bill came in. Thousands of pounds on ISDN calls. They called BT and requested an itemised bill, which turned up in a box...
Turns out that one of the network printers had had its network interface replaced, and the new card was broadcasting its availability across the WAN every 2 minutes. All the lines came up to deliver a handful of packets, and they'd time out and hang up again. Then they'd dial and hang up, dial and hang up, every two minutes for three months until the bill was issued.
Add them up and it's over 85000 calls, all at minimum charge. Ouch