The moment a person erupts with rage due to a vending machine either not having what this person wants or not dispensing it completely leving it hanging on the spiral hook (already paid for.) Said persons have been known to bang on the glass, yell (sometimes including obscenities), rock the machine to try and shake their item off the hook or even break the glass to get their item.
Guy: WHAT?!?! WHAT THE #$%^@! I WANT MY CANDY BAR -- NOWW!!!
Me: Dude it's just a candy bar; chill out.
Guy: SHUT UP! CAN'T YOU TELL I'M GOING VENDAL ON THIS MACHINE?!?!
Me: Here have a dollar and buy another one. Just shut up.
Guy: Whew - thanks man.