That's exactly right. In fact, just the other day it happened to me. I asked for 2 more coins so I could do a coin matrix and they noticed that the coins that were meant to be appearing didn't look the same as when they disappeared. It was kind of embarrassing.
So, I shall never borrow coins...
You're right, you can't use a ten cent coin with a Coke can, which is extremely annoying. I heard that a Solo can works with a ten cent coin, but I haven't tried it yet.
Also, does anyone know where I could get Australian Steel-core coins?