How are the system tests decided? I know that the pretests are very limited due to heavy traffic during a contest, but do system tests really cover all the plausible tricky cases? Now I know there always are hacks for such cases like in educational rounds or even in normal div-2 rounds where there is in contest hacking solely meant to capitalise on such cases, but in the last div 2 round (1087), a submission of one of my friend's got accepted in system testing even though his logic was missing what is arguably the central idea of the entire problem. Namely in problem 2209C, the logic was checking n-2 pairs with different elements and making a triangle between 3 elements out of the 4 unchecked elements in the worst case. A few people submitted checking n pairs and incorrectly checking (1,3) or some other pair and failing on test 9, but the user in question checked (n,n+1) or something like that and (outside of normal 2k,2k-1 pairs) and it passed system testing. It personally took me ~ 15 mins to think of this correction and implement it as I too initially checked just in 1,3 and got Wa on test 9. Like if they covered this flaw for selective pairs in pretests, shouldn't all cases with same flaw be checked before releasing the final standings and ratings?