I wrote a Prime minicomputer emulator in 2005, mostly on a lark because I grew up on them in high school and college and they were pretty advanced for their time, with token ring networking, remote call file systems before NFS, lots of Multics-like features (the Prime founders were associated with Honeywell and MIT).
I announced it on the Prime Usenet group and a guy wrote to say he was interested in using it for production, but had been burned before and wanted to make sure it worked. So I set him up a VM in my basement, he uploaded a bunch of his executables, and they all worked. I charged $1000/mo for the emulator. Since he was in the EU, I knew I wouldn't have a leg to stand on to get paid if they decided to stiff me, so I did some investigating into a Matrix dongle. It was pretty slick, and I coded up a copy protection scheme that allowed the emulator to run only when the dongle was inserted, and it contacted a license server in my basement to verify that it was allowed to run. To make sure my basement server wasn't a critical resource, I had the dongle setup like a battery that got periodically "charged": if the license server was down, the emulator kept running for up to N days on its local charge, giving me time to solve issues with the license server.
This went on for a few years, but they were never happy they had to actually keep paying for the technology. I heard every excuse in the book about why they needed a backup dongle. I knew the real reason they wanted one was to hire someone to hack it, but unfortunately, that would mean removing it, and that would halt the emulator within 5 minutes. So they came up with excuses like:
"Our state regulator requires us to have a backup computer system, so we need 2 dongles". No, you start computer 2, move the dongle, and you're fine.
One time I heard from a guy who said he heard about the emulator from a Prime parts guy in Chicago I knew, and wanted to try it out for a large EU bank. I asked where to send it, and surprise - it was the same city as my other customer! So I call R in Chicago and asked him how the conversation went with this "banking" customer, and R says "It was kinda weird. He said he had a Prime with a failing disk drive, but when I asked what model drive he had, he didn't know. He kept asking if there was any other solution for him besides hardware until R says "Well, there's a guy that has an emulator", which the banking guy jumped on immediately. So I go to Google and lookup this address the banker gives me to send the dongle to, and it's a massage parlor!
All in all, it was a good deal for me and a good deal for him, even if he wasn't happy about it. He was getting paid to support his own Prime software that he didn't have to port. But I learned my lesson as a youngin' about companies saying they will pay for something. I sold some printer/spooler minicomputer software in my early 20's and had a large investment bank (huge actually, as in everyone at the time would recognize their name) try it out for a month. They called and said they were processing the one-time license fee of $1500 but could I give them the access code early so they wouldn't have a disruption for a few days. I did it, and never got a dime from them. Hard lesson learned about being nice.