Investigate a strange occurrence in the combat simulator.
To begin the combat simulator, you will first need to complete the main story. After completion, you will be sent an email with a complimentary pass to compete in the simulator. When you have read the email, proceed to the Armax Arena and talk to the hologram up the stairs and to the left. When you are finished talking to the hologram, head down the stairs, in the center of the room, with a barrier, that says, 'Athletes Only', to a terminal where you can set your options and begin combat missions in the simulator.
After you have played some matches, you will begin to get requests on the simulator terminal, that are just set options that people want you to compete with. Keep completing matches, fulfilling peoples requests and buying the prizes/upgrades from the token stores. Eventually you will get a request from AAA Information Technology Dept, entitled 'Unusual Scores'. You are being recruited to help find a bug in the system and you will need to complete 3 matches with the set conditions to unlock this achievement. The first match you will need to fight super elite enemies, the second match, you will need to complete a match with all of the multipliers turned on and the third match, you will need to complete on the map 'Spin Zone', against Geth elite enemies, with enhanced enemy shields enabled.
After finishing the third match the achievement will unlock.
This achievement was fun to get. If anyone is confused on how to get it you need to play through some combat simulations until you start getting emails from people requesting you to play certain enemies, maps, etc. You can check your emails at the same place you set up combat simulations. One email will be from someone asking you to check for the strange occurrence. Just follow his emails and your journal will update with the assignments.
Found this on the Bioware forums: "You have to get the highest score in the Arena first (9,999 points) which can be accomplished by unlocking Super-Elite enemies with a Gold token or using multiple score multiplier, obtained with Bronze tokens. I also think you have to defeat the Spectre squad (Mirror Match), bt I don't if it's required.
Once you've pretty much done everything, you should get some new challenges in the Combat Simulator called "Unusual Scores". Just complete those as they come up, and you'll eventually get to play the "Fatal Error" mission. Survive that, and the achievement is yours, along with the Fatal Error map for the Arena."
-Complete all the tasks the e-mails ask you to do -Purchase all bronze, silver and gold prizes from the token shop -Complete a spectre match, using the Mirror Match selection (purchased as gold token) -Get 9,999 or above points in a match.
Atleast doing this worked for me. After you've done all these, an email will pop up in the combat simulator selection screen, ''Unusual scores'', follow these and the achievement will be yours.
I unlocked this quest without even buying the mirror match. I did the quests as they came up and this one was recieved after I bought the super elite difficulty mod. All you need for this quest is Geth, + enemy sheilds mod, super elite difficulty, and the Spin Zone map. However to do all the quests before it you need virtually everything else unlocked with the exception of most of the characters and the Mirror Match.