The SpongeBob SquarePants Movie
When they realize that they'll never get past the monster-infested trench, SpongeBob loses hope and cries to Patrick that the two of them are just kids because they eat ice cream, blow bubbles, and worship the Goofy Goober peanut. Patrick convinces him otherwise, stating that they do not worship Goofy Goober, only for SpongeBob to pull down down Patrick's pants and reveal his Goofy Goober underwear that he's been wearing for three years straight.
A little later in the film, during the song "Now That We're Men," Patrick sings that he's finally changed his underwear, resulting in an urchin tearing his pants off to reveal a normal plain pair of underpants.
"Born to Be Wild"
When Patrick is seen putting his biker pants on, he wears his Goofy Goober underwear underneath.