A Raonic-Tomic GS Final (after SFs involving Nadal and Nishikori retirements) where they both retire from the match at the exact moment, leaving no winner. So they hold a LL GS Final with Ferrer and Berdych choking away through 5 epic sets.
In a serious light, anything absurdly improbable like a player ranked outside the top 50 winning a slam would make this board go nuts.
But in terms of what would cause the most chaos from a player who has a sizeable fanbase, it would probably go something like this:
1. Federer beating Nadal at RG (It would be an orgasm that would never end for Fedtards, even twenty years from now)
2. Nadal winning Wimbledon after losing RG (This would probably generate the most hate, vitriol, and tantrums out of any result, the tard mudslinging would be enough to start World War 3)
3. Djokovic winning CYGS (We would see nothing but era debate threads in GM for the rest of 2015 and most of 2016)
4. Murray ruining Djokovic's CYGS by beating him in another 5-set windy final at the USO after Novak wins RG and Wimbledon (Djokotards would be in complete ruins, Giftovic would become Novak's official title for the rest of his career, and people would call the match the death of tennis)