I went to a school with many friends who experienced with hallucinogens regularly. But they always followed these rules:
1 - don't look at a the mirror
2 - cars are for real
3 - you can't fly
1 - don't look at a the mirror
2 - cars are for real
3 - you can't fly
I've also heard #1 several times. I've also seen people break that rule. Their demeanor changes as something dramatic is hallucinated—the mirrors definitely caused immediate reactions in the people tripping, but that demeanor change was quickly replaced by other hallucinations, interests in the room, etc. The overwhelming consensus was very positive with respect to the whole trips every time I've seen it.
What would the person see?
I just kept staring at my eyes. they kept getting more and more blue, and more and more deep, and some sort of strange peace came about me and my body all over. I knew myself. from that experience on, my whole way of thinking changed and I decided to stop doing stupid shit. since then (been a few months now), I've only been able to focus on getting what I wanted out of life -- for myself and everyone around me too (no matter how awful they might be). I like to say encouraging and uplifting things, only rarely do I see the world in sadness. I can quickly get out of sadness now too. it's all perspective after you've seen without filters
I highly recommend shrooms. prepare though. be mentally ready for a mind expanding experience. if you do meditation, do it in a dark room, and try to get as deep as possible before the effects kick in. and then later come out if you want, but always remember you're still you, just you without a lot of those extra filters (that you yourself have constructed!).
Not a pretty sight.
You see your reflection -- obviously. The problem (potential problem, it can also be totally fine), is that because of the visual tricks the drug plays on your, you can see it swirling, melting, bending, etc.
The other aspect is that, seeing your face can trigger self-reflection, and you can start having bad thoughts about you, how you're older, ugly, seeing aspects on your character thay you don't like, thinking shit you've done in the past, etc.
If you're not that kind of miserable person in normal life (or with hidden but known to you pains) you'll be just fine looking in the mirror.
As an aside, no you won't suddenly see a monster, snake or such BS -- you'll see what you see + visual effects like twirling, shine, kaleidoscopic lights, etc.
In retrospect, this kind of pat, simplistic advice has the ring of either inexperience or lack of self-knowledge. A person who is in touch with himself and his own subconscious will not avoid looking in the mirror (either literally or figuratively).
One of the most important things one can learn on psychedelics is how to deal with unpleasant aspects of yourself, others, and the world around you. One often learns much more from "bad" trips than good ones.
If one goes in to these experiences with a sincere determination to learn from whatever one may encounter, to use one's time constructively, and with a great deal of honesty and humility, then half the battle is won. Then looking in the mirror can be seen as an opportunity for self-discovery, not as a potential unpleasantness to be feared or avoided.