Here's A Nice Method To Get Cool Coins!

I have done something similar before. The vending machine didn't give out different change if I pressed the change button but I noticed if I put more change than I needed for whatever I was buying the change I got back would be different, so I would load up all the change I had on me and exchange it for new stuff to look through. Have never really found anything special doing it yet but that doesn't deter me from continuing to do it.

I used to load lots of extra change (usually, a bit less than what was needed, then a banknote) into the machines at the metro (underground subway) that sold the single-trip tickets, back when these tickets weren't ludicrously expensive. These machines gave change in 1 and 10 ruble coins, and I got a few nice commemoratives that way (especially the 1 ruble, which was new at the time).
After the single-trip tickets randomly became nearly twice as expensive, I stopped using these machines at all, because it really wasn't worth it; getting a 5 trip ticket and hoping to use at least 3 of these trips (it's valid for 90 days) was cheaper.

The problem is, the only vending machines I ever pass are the ones at the high school I go to to learn Chinese, I don't even go to that school.