From Regular Show Season 4 Episode 15 "Do or Diaper"
In case y'all don't get it, Mordecai (the blue bird) makes a bet with Muscle Man (the green guy) & Rigby (the raccoon) that he can kiss his crush Margaret (the red bird) by midnight, and if he doesn't he has to wear a diaper for a week. This being Regular Show, Mordecai takes too many mints causing a snowstorm to unleash throughout the park. Margaret eventually finds out about the bet and ditches Mordecai
Mordecai apologizes for this, and Margaret gets after him for why he's always acting so weird around her. After nearly freezing to death, he admits that he likes her. After some thought, Margaret asks what time it was. It is 11:59 PM, they have one minute to kiss.
Margaret then warms up to Mordecai and holds him thus making the storm go away and the snow to melt (symbolism on how the warmth of Margaret's heart melted the snow???). They then proceed to kiss which is then followed by Margaret fucking him over at the last second whispering "Have a nice week, diaper boy!" thus causing Mordecai to lose the bet.
This was done for three reasons: 1) Margaret wanted to teach Mordecai a lesson about using girls and not treating them like living things but rather objects, 2) Margaret wanted to be an ass to Mordecai in a friendly way, and 3) to keep the trope of Mordecai never achieving Margaret as his girlfriend well and alive for another season.