Mythruna

Mirrors would be an awesome addition to the game, mainly after player modelling is created. They would be good as thinner surface plates (similar space usage to minecraft's slabs) that can be placed on walls, floors and ceilings/rooftops giving off a reflection from the mirrors point of view. I've seen it done before on map mods for games, but not in Battlefield 3 by EA. I have no clue how to create it, except for the fact that the mirror model is assigned a texture, then the reflective property is assigned by a shader.

TL;DR: Mirrors would be a good addition as a slab feature. ;)

Every mirror plane requires the whole scene to be rendered again from that perspective.  Even worse, things that were clipped from the regular camera may not be clipped from the point of view of the mirrored camera and so you end up even rendering lots of different stuff.

ie: it's very expensive, performance-wise.

Worst case scenario: some wise guy puts two mirrors together and crashes the game. I see the problem now

Portal can do it though.

If you put a portal in front of you and behind you then how many portals can you see through before it stops?

How many portals can you place in Portal?  I thought it was just two.  That's a few less than thousands and thousands. ;)

I believe it stops around seven layers, and you can only have two in Portal 1. It kinda fades out into fogginess, so the transition isn't to obvious.

I have a mirror on two of my walls that are parallel to each other, the amount of "portals" I see is quite infinite.

In the video game Portal?
Or were you not trying to correct me?