The worst luck I've had so far, knock on wood, was trying to breed a female Timid Snorunt that knew Spikes. I accidentally bred a bunch of Cacnea, then had to go catch a male Cacturne that knew Spikes, breed it with a Roselia, then breed that offspring Roselia with the Timid Snorunt I bred earlier. I hatched about seven Snorunts, five of which were the wrong nature, one of which was a male, and on the last egg, got the one I needed.
I also plan on Masuda-ing my Japanese Feebas for a shiny one, but I can almost guarantee if/when I do get a shiny one, it will have some terrible nature.