MSK changed from UTC+4 to UTC+3 on 26 October (This has nothing to do with DST). This change affects contest time from the next contest (Codeforces Round #276). If you believe MSK is UTC+4, you may be upset. Pay attention!

P.S. By "This has nothing to do with DST" above, I want to say this topic is not equivalent to Daylight saving time ends! the change of contest time is not because DST ends (or begins), and this change is permanent. Sorry for my poor English.

If a window for viewing a submissions history or hacking a solution doesn't open even if you double-click (or ctrl+click) in a cell, you may be able to solve it by clearing your local strage (e.g. localStorage.clear(); in JavaScript console).

In my case, QuotaExceededError indirectly caused this problem.

As you might know, stderr doesn't affect a judgement. Therefore, if you use stderr instead of stdout for debugging, you won't get WA even if you forget to delete (or comment out) the lines for debugging.

Of course, you can never get AC if your solution is wrong in the first place :)