I don't have a lot of personal stories yet. But...
Was presented with a Megatouch Gold machine that had booting issues. Tried a number of things with no joy. Finally discovered that the RAM wasn't seated correctly, and presto! Fixed the machine. Or so I thought...
In the process of fixing the machine I'd used a brand new riser card that we'd gotten from NECO. It had the same result, so I put back the original. Once reassembled, I left it alone.
Boss man said he wanted to send the machine to a bar about four hours north (along with other equipment) and stated the machine was ready. I, along with everyone else there, playtested it, saw it was working, and included it on the truck. It went north.
Several hours later, the guy that brought it north revealed something: it wasn't taking coins! A machine that won't take coins on location is a useless machine to an op. He spent at least a couple hours on it before declaring it unrepairable on site, and brought it back.
Long story short, it turns out that riser card somehow shorted out the coin circuitry on the board. And the boss decided to swap out cards to test things out, and ended up shorting the board on the only other board we had, but at least determined what had caused the problem. There was another board to be used from another machine (just had to swap the license keys) and that worked fine (of course, before doing that, he made sure to ditch that flakey riser board). But that doesn't fix the problem of the two boards that won't accept credits.
So now what to do? Send out the board? Likely it won't even make the money that it would take to repair it. Buy another board? Same problem. Troubleshoot it in spare time? Sadly, this is likely the only option. I have my suspicions as to what components were toasted on the board, but without schematics I can't confirm. And the boss doesn't want anyone touching the boards now except him, so I'm guessing they will remain in stasis for at least six months.