The Yandex.ru webmail monitors itself and sends information about certain unexpected error conditions back to the server.
Most servers would just shrug and record the error. However, mail.yandex.ru seems a bit more..secretive about it. Perhaps because secrecy is a Russian thing, perhaps to amuse browser QA, or perhaps they just try to take web debugging to another level..? Here is the full text/plain document they return when the script says "Oops, an error happened":
Ssshhhh! Don't tell anyone.