Silly mistakes are like syntax errors. They can cause your entire program to become useless. Pity that there is no compiler for detecting them.
So whats the silliest mistake you have ever done in a contest problem ? And when you realized it later, you had that "WTF was I thinking then" kind of feeling.
Yesterday while doing div 2 B, I was pretty confident that my approach was correct, but kept failing a pretest.
I couldnt sleep properly due to that mistake, so I woke up at 3am and found this stupid silly mistake.