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Speed is decided before the turn even begins, so Sableye's Prankster speed beats Banette's non-prankster speed, even though Mega evolution occurs. The next turn however, Banette's status moves will outspeed Sableye.
In a doubles battle, my opponents Scrappy Exploud used Boomburst. It didn't affect his ally Trevanant, but did affect and damage my own Trevanant and Cresselia.
Is this a bug? Is Scrappy Boomburst supposed to hit all ghosts, regardless of if they are ally or opponent?