sometimes during a contest when I think a problem have tricky test case(s) I go to my room and start searching for solutions which doesn't handle those tricky cases but sometimes after I read about more than 15 codes I still can't find even single solution which doesn't handle one of these tricky cases then I start to think: am I unlucky because I am in a room where all its contestants are very careful for tricky cases?

but then after the contest I discover that all such test cases exist in the pretest so if a solution didn't handle one of them it will not pass the pretests, so I just realized that I wasted my time while reading the solutions.

that's why I want to suggest a new feature in codeforces round rules that makes us able to submit solutions to a problem even after locking it so we can submit solutions that doesn't handle those tricky cases so if it passed then we know those tricky cases doesn't exist in the pretests , ofcourse those submissions doesn't affect our points and only last solution submitted before locking is judged in the system testing.