I mean the aim is luck it seems. Always seems there is one random miss or one random slider break that ruins the FC even though you've gotten 99% on the song 15 times already. Then after you finally FC it you can't FC it again.The infamous 1xmiss. The notorious sliderbreak. I can relate completely.
Thelewa posted it somewhere, and I can't find the thread, but he wrote that the misses are because of a lack of focus. In hindsight, almost all of my 1xmiss/sliderbreaks were because I started spacing out, or thinking about something else.