I managed to score a complete, boxed NES Action Set and a NES Advantage from a coworker last year. The console looks like it hadn't been opened in several years, and I cleaned the interior of the system with an air blaster prior hooking it up. I also gave the carts that came with it (Mario/Duck Hunt, Super Mario Bros. 2, Golf, MTV Remote Control and T&C Surf Designs) good alcohol swabs on the contacts before hooking it up to a television. I'm using the composite outs on the unit.
Over the last few months, I would say it successfully boots a game one in ten times. I believe the MTV cart (a Hi-Tech joint, the very definition of quality) to be somehow fried, so I've called those boots a wash. Otherwise I've been able to get the other four running very erratically. I got through nine holes on Golf shortly after I picked it up, and everything else was playable after a few tests. Fast forward a few months and I decide to start tooling with Super Mario Bros. After struggling for about half an hour with dusting, swabbing, checking contacts, and trying various combinations of inserting and compressing the cart (getting that "ka-TING" noise), I got into the select screen and started a game of Mario. Then in 1-2, something happened that was so bizarre I wish I had captured it. A fireball, the sort that jump out of lava pits, jumped through the stone floor a little after passing the mid-point of the level. Now, I played me some goddamn SMB when I was younger, and there is no lava in this level. None. For some reason, the NES realized this assault on my senses was too much and decided to hard lock at that point. Couldn't boot the cart again after a few tries, so I went back to Golf. That worked on the 18th attempt.
Now, I will grant you that Golf and the system running it are approaching their 30th birthdays. Even with that in mind, however, I remember playing NES games in the early 1990s being dodgy. One of my best friends in elementary school had a copy of Bigfoot that would only run on his NES after putting in the cart, not compressing it, and slapping the top of the console. There also seems to be a wide range of tolerances for the contacts between the cartridges and the console itself, such that there is in fact no universal "correct" procedure to get a NES to play a game. Has this always been the case, or is it just getting worse? If so, what does that say for the long term prospects of archiving the NES outside of emulation?