There is a change counter at a local supermarket. (I go there almost every work day for the salad bar.) This machine seems to be broken down far more than operational. I’ve seen the repair man there many times but a day or two later its out of order again. Are those machines inherently unreliable? If so, why? As an aside, my bank had them and they were free for customers of the bank. Then someone tested their accuracy and found that they were pretty consistently under-reporting the total at multiple branches. Now, they have been removed from all branches.
My first guess answer would be that entering non-coins into the machine, such as slugs or washers, would cause things to break. Engineers can test for all types of coins, but there is a huge array of non-coins and it is impossible to test them all to see what happens.
A friend of mine repairs and services coin counters. A lot of dirt gets into them, anything stuck to a coin, whatever was in your pockets, lint, hair, bits of paper, fingernails, candy, eventually clogging up the works. Some of them are quite old also and the parts just wear out eventually. And then dimes can be a problem, they’re so thin they get jammed in between parts.
Those coin counters are actually pretty reliable, not much different at the core from those used by banks. Then again, bill counters and stackers are also extremely reliable at a bank.
Put either in the consumer world (bill acceptors on vending machines, supermarket coin sorters), and all bets are off.
For bill acceptors, generally the worst quality bills are inserted to get them out of peoples wallets. They are torn, severely worn, folded, dirty, etc. It’s almost a miracle that bill acceptors work at all.
Coin counters (even coin slots on vending machines) have it even worse. As mentioned above, in addition to the dirt, and yes, damaged coins that people toss in, there are usually coins covered in everything from oil to glue. There are foreign coins. There are washers, screws, nails. There are matchsticks, lint, pieces of paper (really bad in coin counters), pieces of cloth, cigarette butts (yes, people as a whole are pretty filthy when they don’t own the equipment), hair, food… well, you get the point.
** Years and years ago, I worked for a coin-op company, and then a banking systems company. I’ve actually seen all of that in publicly accessible coin counters. Some were just plain disgusting to try to repair.