Playing the hidden, randomized VS CPU mode in the unlockable game Street Fighter Alpha 3 Upper in Street Fighter Alpha Anthology on PS2.
You get to the hidden randomized VS CPU modes in the Anthology's games by highlighting "VERSUS" on their main menu, then--in the US version--hitting X followed IMMEDIATELY by hitting Triangle and R2; if you did it right, instead of landing on the character select screen, the game will go straight into a match, with random characters. You can change the randomization of characters and ISMS/modes from the pause menu.
One tiny drawback to this mode is that the stages aren't randomized--it always runs through the game's stages in the same order, each time. That can get a bit boring, but I found that setting the battles to be just one round helps in that regard, since you're speeding through many more stages that way, at least. : )
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