The ceremony in question took place in San Ysidro Ranch in Santa Barbara, and while the location is close to where Katy grew up, the bride and groom-to-be were strangers.
A guest told People magazine: "Katy and Rob were just sitting next to each other watching the wedding rehearsal in the main courtyard area."
It's clear the pair hit the wedding rehearsal unplanned as they were both reportedly dressed down for the occasion, with the source describing: "[Katy] was wearing a hoodie and big sunglasses. They just sat together and watched."
Well, we all love a good wedding, so who can blame Katy and Rob for stopping by!