I ran a small ISP years ago. We provided a backup network connection for a local hospital - they had a few T1s to far better providers, but connecting to us was cheap and gave them a bit of redundancy in a place where getting redundancy was hard.

One morning, we discovered that the connection was fully saturated. This was highly unusual, so we called the hospital to make sure everything was ok. Turns out, they were in meltdown mode - nothing was working correctly and they couldn't get in touch with their head of IT; he was on vacation out of the country.

We came over to try and help and, after working with the small onsite staff, found the problem. A core application they used was throwing all sorts of errors because the disks were full. That was the cause of the problems in the hospital. That still didn't explain the network being saturated.

The disks were full because the missing admin had set up an open FTP server, where anyone could upload or download. It had obviously been running for a long while, but while he was out, it got really popular. The thing was full of porn and warez and all sorts of things you don't want on your network, much less the equipment that should be managing healthcare records.

We got law enforcement involved. Initially, we thought the FTP was just poorly configured, but after some digging, it seemed clear that the admin was aware it was there and had set it up that way on purpose. When the admin returned from vacation, he was stopped at the border and was searched. On his laptop, they found several of the same files that were on the server, including some child porn. He was arrested at the airport.