It is always when you sim 15+ and get nothing out... and thats not uncommon. Although I also started to write my farm down and came close to the percentage rage, I started to think about when there is the extremly bad result again.
It is probably only around the focus: If you are on 99/100 and you do not get the last one in the tries, you forget all the mathematics and think on the conspiracy. If you are on 7/30 and you get 1 out of 1 try, you might not recognice that you are at the 100% rate, you only think "just" one.