I have confirmed the existence of this bug myself today in ReGBA, and it is indeed caused by Power+X. Though it would occur similarly in any application, not all applications have all keys bound to an in-application action. That's why I used ReGBA.
Here's my test setup.
a) Open ReGBA alpha 3. It has Rapid A and Rapid B bound to Y and X.
b) Open Pok?mon Emerald. (Any RPG with menus will do, but hey, I like Pok?mon.)
c) Raise Power, press X, lower Power.
d) Walk around on the map using long presses of the D-pad. [Expected: the character walks around the map, and nothing else gets the button press. Actual: the character walks around the map and some Power button functions are activated.]
e) Get into a Pok?mon battle, wild or against a trainer.
f) Press A to enter the FIGHT menu.
g) In the FIGHT menu, press D-pad directions, quickly or long presses. [Expected: the move selection changes, and nothing else gets the button press. Actual: the move selection changes, long presses send X to ReGBA and exit the FIGHT menu because Rapid B is bound to X, and some Power button functions are activated.]