@Linkat,
You are a lucky one. I have never dealt with a vending machine which had an envelope to request a refund. Machines I've dealt with have gladly taken the coins and then just sat there looking at me with all their buttons and pullers. As I stand there staring at my desired product I am forced to notice the terrible infection which has started in on my fingers as they were cut and then subjected to deadly microorganisms at the coin slot. Is it any wonder I curse the machine, slap it, punch it, kick it? Am I not human, do I not deserve a back of M&Ms? And who had the insane idea of making the front of these darn machines so reflective? As I stand there looking at the nutritionless product that I want, I notice my expanding waistline, is it any wonder I curse the machine and slap it again? As I pass out from the gangrenous infection, is it a war as I grab the machine one last time and it falls upon me and I rip it's electric cord from the socket? If it kills me, don't I indeed have the right to return the favor or at least make it suffer for a time?
No, I never had the great fortune of a machine with a refund method attached. I did however once work in a place where myself and a young lady were in charge of the machines. Whenever a person lost money one of us had to go to the machines, open the culprit, and hand the item of choice to the employee. (we also had to load the soda machines, which was fun. The snack machines were loaded by an outside source.)