Perry tweeted her followers "[Calvin Harris] will NOT be joining in on the fun and has CANCELLED last minute (sic)".
Harris then re-tweeted that, following it up with the explanation that "[Katy Perry's] team suddenly moved the goalposts, and i was to appear on stage with no production".
Unsurprisingly, Perry didn't take it lying down: "…funny the goal post seems to be perfectly fine for New Young Pony Club, Yelle, Robyn, Marina & The Diamonds to name a few."
Harris then tweeted, "I am really sorry @katyperry I'm just upset because I really wanted to play but ur team made it impossible. Didn't intend to cause offence or upset anybody."
Well that's that sorted out then.