As you people may or may not remember from the original Adventure Time episode called Burning Low, according to Princess Bubblegum, when Finn kissed Flame Princess, her elemental matrix became unstable, and extreme romance, like kissing, can cause her to react so violently that she could burn a hole through the core of the planet. The resulting explosion sends Flame Princess falling down a hole, burning everything in her path, and Finn jumps in after her. Jake and Princess Bubblegum then work together to cut off the oxygen to put her fire out. In the end, to prevent further damage, Finn and Flame Princess realize they can't kiss directly and instead kiss the same rock to represent them kissing. Phoebe's natural elemental power is in a precarious balance. Her intense, romantic interaction—specifically the kiss—acts as the catalyst that triggers this instability. The physical contact and strong emotion cause her power to erupt in a rapid, self-sustaining reaction. As Bubblegum puts it, this could cause Phoebe to "burn a hole right through the core of the world" due to the intense, uncontrolled heat. Phoebe's fire power was at several hundreds of degrees Celsius or several hundreds to over 1,000 Kelvin. Any hotter, and she would approach supernova. Phoebe is like the temperature of the sun. So not only could Phoebe nearly killed everyone, including herrself, but she could set fire all across the Land of Ooo and destroy all life as the people knows it. Apparently, kissing Phoebe caused her fiery powers goes supernova because coming into direct contact with her lips or applying force must start some kind of a fiery chain reaction, thus melting the hole through the planet's core and destroying it in a process.
As you all know, when Flame Princess gets exposed to extreme romance, her Elemental Matrix will becomes unstable, and she would burn so hot that she will melt down through the planet's crust. Then she would be thrown by gravity back and forth until she burns the world from inside. In "Burning Low," Finn kissed her, thus making her Elemental Matrix become unstable. However, the process was stopped because of the lack of oxygen, and Flame Princess was burned out before she could burn the whole world. However, this may not be a power, but a condition or disorder, since Flame Princess does not manipulate it or permit it to happen.
