Murphy's Law. The effect was theorized by Baljeet after Phineas and Ferb researched Milo in "The Phineas and Ferb Effect".
In order to gain an upper hand at the Pistachions, Phineas and Ferb were offered to let Milo go through some tests, as Murphy's Law has not been researched before. It turns out from the research that Milo has a natural field of negative probability ions surrounding him, and that likewise, Phineas and Ferb have a field of their own as well, but filled with positive probability ions, which may be the reason why Phineas and Ferb never get in trouble by Candace. Baljeet dubs this phenomenon the "Phineas and Ferb Effect".
According to Baljeet, positive and negative probability ions cannot occupy in the same space, so if Milo stood in between Phineas and Ferb, Milo's negative probability ions would travel elsewhere. This could work in Milo, Phineas and Ferb's advantage if they made a device that allowed them to aim the negative probability ions at any desirable target, which has become the basis of the Murphy's Law Suit.
- It's possible that Phineas and Ferb are affected by the opposite of Murphy's Law, the Yhprum's law. It states that "Anything that could work, will work".
- This phenomenon gave name to the crossover.
- It should be noted that despite Phineas and Ferb's huge fortune, both have suffered notable misfortune during their several of their adventures (e.g. in "Phineas and Ferb Summer Belongs to You!").