You know sometimes when there is a mirror there and at first you don't realise it's a mirror? Can be quite confusing, maybe even disconcerting for a second or two depending on how well it is disguised. And that experience is well known and almost too obvious to mention... however, in this thread I've not seen anyone mention the opposite experience which can be equally odd.
I'm thinking of that pub just south of Dalston Junction, is it called The Haggerston? The toilets are at the back of the pub with a door into the boys and that for the girls next to each other, with the result that the two rooms run next to each other and share a common wall, which is where the sinks are and so on. Above the sinks is a mirror, except after a double-take you realise it's not a mirror, and you haven't got prettier, but instead you are looking across into the face of someone in the girls washing their hands. I've been there a few times and it always catches me out cos it's exactly where you expect there to be a mirror, and there is someone where you expect your reflection to be, and chances are they are doing the same thing as you so it works rather well.
There also used to be a pub in Camden with a similar thing going on, but I think it was above and behind the bar. One expected there to be a mirror there, but instead there was a hole in the wall with a view into the other bar.
I find something quite neat in this cos mirrors can be used to trick the eye, bamboozle the mind and create all kinds of optical illusions - and yet in this case someone has used the absence of a mirror and the fact that you see nothing at all out of the ordinary to trick your mind just as much.