Ok, firstly I would like to tell you not to be silly and become afraid of looking into mirrors.
If you were born with the ability to see into other worlds, perhaps you should be more cautions, but if you were born with this ability then you'd know by now and the mirrors would be a picnic!
Actually, fearing seeing yourself in a mirror is a pretty good symptom of being possessed by a malignant or unhealthy spirit (it is the spirit you know is inside you which you fear seeing). My theory goes that storybook vampires don't have a reflection, not because they don't have a soul, but because they won't look in the mirror. I think that the stories were based on accounts of people who were possessed by spirits, the thirst for blood was not that of the human vessel but that of the spirit... just my theory, anyway...
As a young child I developed a technique for looking "through" things in order to see the spirits in them. I found that I had the ability to see spirits but could not control it. found a way of looking at things which would reveal spirits to me. Actually, as a child I could never see those 3D things with a picture hidden in them because I was always using this technique instead of simply de-focusing my eyes as others did. It was just my "go-to" way of seeing the true nature of things around me.
One night I thought it was high time I saw who I really was and so I looked at myself in the mirror.
What I saw terrified and traumatised me for a long time afterwards.
I will do my best to describe it to you now: It was a malformed foetus who's skin hung loosely, weathered and ragged with deep wrinkles like those you'd expect to see in someone who was over 200 years old. The nose was small and not properly formed yet, the mouth was slit-like and slightly hare-lipped. The creature's skin was totally drained of all colour, it wasn't even white, it was pallid. The most pronounced feature of the entity I saw was its eyes. The eyes were huge and black without iris or white, they cast no reflection. From its eyes shone darkness - well, not exactly darkness, more like "anti-light" it was a kind of darkness which extinguished light, the eyes radiated a whole spectrum of "un-colours".
It was not just the creature's appearance which frightened me, it was the belief I had that I had just seen either a spirit which was possessing me or my own true nature. I saw this creature several time again in the mirror and other creatures, some nearly as nightmarish. I did not realise that I wasn't seeing myself, but rather that this was my first experience seeing the entities who can be seen in mirrors.
I have done research on these beings since and found out only a few things.
There was a writer - can't for the life of me remember who, the first one who came to mind was Rudolf Steiner, but I'm almost certain it wasn't him, could have been Sax Roamer(?) - who spoke about how the reflection we see in one mirror is not a "true" reflection of the world, but the reflection we see in two is - I don't think he was talking about how the image we see is "mirrored" either, but from the context it seemed to imply that he felt that in looking into a mirror we thought we were looking at our world but we were actually looking at another, through the reflection of the reflection we were able to see our world again.
The consensus is that the entities we see in the mirrors are not actually in the mirrors but in other worlds and they use the mirrors as windows or even doorways into ours, or we use the mirrors in the same way.
There are countless references to this in popular culture. I think Graham Masterton wrote a story about people being stuck in the mirror world and then there was that movie (which had nothing in common and wasn't that good anyway) - my favourite are of course the urban legends. Bloody Mary, the Three Kings, the Five Questions, etc...
Stories of people invoking beings in the mirror or visiting magical lands abound in folklore and fairytales. I don't think I need to name examples here
Not all the creatures we find in mirrors are frightening or dangerous, but as with all things, and as the fairy tales (modern and ancient) caution us, it is better to be safe than sorry.
I am very interested to read more on the beings who can be seen in mirrors, if you have links to websites, academic writings, recommended books or stories of your own to share, please don't be shy!
And remember, of all the things you could see in the mirror, the scariest of them all isYOU