I remember one of the first applications I worked with, at the beginning of my career was built using Clipper (for those who have not been on the area for long, here's a pointer to the Clipper language, pretty cutting-edge at the time :) ).
Clipper used the same database architecture as dBase did, and databases were stored on regular files on the file system, and for each index you would need an accompanying file, those were the .DBF (database file) and the .NTX (the index file).
Every once in a while the database file and the index lost sync. That could happen for a couple of reasons, one of them being forgetting to open one of all the indexes for a given table (represented by a single file).
When that situation happened, the user was presented with a red screen with yellow text saying: "Corruption Detected". Imagine the user of a financial software presented with the corruption detected message, with a red background :-) Some calls we got at the time were pretty fun.
For another product I worked on, a SMS game that resembles a Tamagochi, where the user interacts with others by taking actions, like dating, going to the gym, going to the club and so on... I remember that when we had an exception in Java (the backend), we just replied with a generic message: "Ok, you got it! What do you want to do now?". For the user, it seemed the game understood his last action, but we obviously had some processing problems.
So, on the mobile operators, most of the users who did UAT for the product would call us and say: "When I attempt to chat with someone else, I am getting that 'Ok, you got it' message" :-)
Now it's your turn: do you remember any known error message among the applications you worked on?