My submission #84516509 for question B of Codeforces Round 651 (Div 2) passed the pretests but failed the system test.

From what I understood, this usually means that successful hacks were presented during the hacking phase/ competition itself and my solution fails on the hack cases.

However, from what I can see, there are 0 successful hacking attempts for this problem throughout the course of the competition.

Can anyone shed light on what happened (solution passes pretests but fail system tests with 0 successful hacking attempts) ? It is quite discouraging (the bug was minor which resulted in runtime error, and I promptly spotted and fixed it as soon as system testing is over) that I was initially expecting a delta positive from this contest the moment it is over, but judging from my ranking after the system test, it is likely to be a delta negative instead.

(P/S: Needless to say, I did not use any randomised algorithm/ functions which would result in a different output. All output were exactly reproducible and is dependent on the input only.)