What is the strange occurrence in the combat simulator?

There is an achievement in Mass Effect 3 : Citadel called Technical Issues:

What is the strange occurrence in the combat simulator and how do I find it?

Solution 1:

In order to get to the combat simulator, you need to beat the main story of the DLC.

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.

From there, you will get various requests after completing some matches, and it is the "Unusual Scores" request that leads to discovering the strange occurrence, and thus getting the achievement.