DIP switches inside arcade cabinets
Many years ago, I went to one of those weekend carnivals run by a church. It was the sort of thing where you can go for a walk, get a hot dog or a burger, grab a drink, play some dubious games, and generally have fun. It was something they did every year, and everyone from the neighborhood showed up at some point during the weekend.
One year, they had a room open which was full of video games. This was back before the days of nothing but fighting games, so there was plenty of variety. I saw something interesting near the door: a raffle drawing for a real arcade game! For something like 75 cents per entry, you could buy a ticket. I had $1.50 with me, so I bought two, more for random amusement than anything else.
You didn't need to be present to win, so I wrote down my info on the tickets and put them in the box. Imagine my surprise when I found out a couple of days later that I had in fact won a full-size Donkey Kong 3 cabinet. A couple of strong guys put it in a pickup truck and ran it down the street to my house, and now I had my own video game!
It quickly became apparent why they had given it away. Someone had apparently gone to town on the front panel with some kind of crowbar. The original lock was beyond repair, and they had attempted to secure it by adding a huge metal bar which could be padlocked. They didn't give me any keys, but fortunately it was not locked. I was able to just open the door to recover my quarters.
Eventually, I learned a few things about it. There was a big square button inside which had the effect of giving you a credit. You could also use it to do some kind of service stuff if I remember correctly. You couldn't change any settings from there but the game itself worked fine, so I went with it.
Then, one day while snooping around inside, I noticed a pair of keys hanging on a hook not too far from the front door. They were the typical cylinder thing you'd see on video games. I managed to fish them out and found they fit in the lock on the *back* of the case. One turn and a pull later, I had the whole thing opened up.
Now things started getting interesting. On the inside of the panel I had removed, there was a sheet which talked about DIP switch settings and all sorts of interesting options. You could change the number of coins required for a credit, how many lives you got with each game, the difficulty, and that sort of thing.
By this point in my life, I had a bit of experience poking at computer hardware courtesy of my UART upgrade, so I went for it. I twiddled all of those switches until I had a game which would run for a very long time: many lives per credits, and many credits per coin. With that done, I closed it up and returned the keys and just used it normally.
There wasn't much more which could be done with it. The cabinet didn't have the full set of controls you would need to hook it to a NES (which was the newest thing around at the time), and I wasn't about to start chopping holes and soldering things, so I just left it as-is.
There's only so much fun you can get out of running around with a tube of bug bomb even if it does mean pushing smoke up an ape's rear end. After a couple of years, we sold it to a neighbor for some token sum. I have no idea what happened after that. For all I know, it may still be out there somewhere running with those same crazy settings.