A remix of the term, "Party Foul".
Usually an exclaimation from a social gathering when something goes wrong. So, any comment or action that results from someone screwing up, or, just being stupid, will receive an "OH, FIESTA FOUL!"
Brian: Bro, Kevin just hooked up with Austin's girlfriend
Taylor: Oh shit, FIESTA FOUL!
Kayla spills beer on Lindsay's new white shirt.
Alex: Fiesta foul!!!