Quoted from LyonsRonnie1:

You said you paid $20 each way and $20 at the Museum, playing quarter games. One game was slanted (but you could still play it), One game died in the middle of the last ball after you had played most of it, and one game took your quarter.
So you lost 3 quarters (we'll be generous) out of 80. And you, years later, still complain about it when given the opportunity.
If I were Tim, I'd sell off the collection and go out of business. Tim won't do that, though, because he's more concerned about raising money for charity, and keeping Pinball alive than he is pleasing every customer 100% of the time.
This is a great example of why older machines aren't operated anymore though. At least with a modern machine the coin mechs are more likely to work properly and the game likely won't reset. If Tim only operated modern machines, he might be able to get his satisfactory game level up from 77/80 (96.25%) to more like 78/80 (97.5%).