Princess Bubblegum's Rock T-shirt is black with a yellow snake coiled around what seem to be two muddy Candy People heads on stakes. Marceline gave it to Princess Bubblegum as a gift. Jake mistakes it to be Marceline's when they recover their things from the Door Lord in the episode "What Was Missing," but it is claimed by Princess Bubblegum, who says she wears it all the time as pajamas. After retrieving the shirt from the Door Lord, Princess Bubblegum tells Marceline that the shirt means a lot to her.
In "Sky Witch," Princess Bubblegum gives it to Maja in exchange for Hambo. According to Maja, the shirt has "fresher" sentimental value than Hambo. Maja then wears the shirt (minus the sleeves and strips cut off for her potions) in "Something Big," where she uses its power to awaken the ancient creature Darren and attack the Candy Kingdom.
- At the end of "What Was Missing," when Princess Bubblegum is seen running away, her shirt is colored magenta instead of black.
- The fact that the shirt means so much to Princess Bubblegum causes Marceline to blush, and the "I'm Just Your Problem" song contributed to the Mathematical! Controversy. The subsequent opening scene of Bubblegum sniffing the shirt when she wakes up in "Sky Witch" deepened it further.
- Marceline is seen wearing the shirt in the "What Was Missing" promo art.
- There are several small differences between the real-life shirt and the one that appears onscreen:
- The sprinkles on the left head are brown instead of red and blue.
- The stake through the left head is shorter and more rounded.
- The green "fire" in the background is more stylized.
- The snake's eye is different.
- The snake has eight teeth instead of ten.
- In the Mathematical! video and in the storyboard for "What Was Missing," the shirt design has 3 heads and no snake.
- In the episode "Sky Witch," Princess Bubblegum can be seen wearing it as her pajamas at the beginning of the episode. She then wears it under her coat for the rest of the day.