The Bubble with Billy - S4.E22

When Billy desperately wants chewing gum at school, Grim gives him a bottomless gumball machine, but warns him not to swallow the gum. Billy ignores Grim's warning and starts swallowing the gum, causing him to constantly burp bubbles that fill the entire city. Mandy and Grim try to get Billy to cough the remaining gum that he has in his stomach, but instead Billy swallows the entire gumball machine. Billy coughs huge amounts of gum all over the city and ends up with a supernatural rash from another interdimensional world.