I don't use mirrors... only because I'm only just getting our first projection effect together. I can see a potential use, though. Depending on the distance you need to have the projector from your target (screen, frame, skull, whatever), you can locate the project at 90 degrees, and use the mirror positioned at 45 degrees to turn the corner for you. never tried it, and obviously the image gets reversed in the process so you'd have to take that into consideration.

 

There are some issue that mirrors can cause. First is, they are not all created equal. Imperfections in the surface will be translated into your image, and if it isnt a first surface mirror, can create a double image. They also slightly reduce the images brightness. One benefit is that you can reduce your overall throw distance by using a mirror.

 

I have not used a mirror, but I HAVE used a pane of glass to create a 'Pepper's Ghost' illusion in a haunt before. The 'ghost' worked amazingly well, but you have to be extremely careful with the glass and ensure the safety of your guests. It was a major hassle to do that trick, but it was well worth the end result. If I can find a pic of my ghost I will share it - but it was hard to photograph because it was always in motion and I was taking a long-exposure in low light, so the ghost is a bit blurry. I'll look around and see if I can find it.

 

I haven't used mirrors in my yard haunt, but I've experimented with a small disco mirror ball. It multiplies the image. And if it's a rotating mirror ball, you've got a swirl of rotating images. The following was taken using a cheap Discovery Wonderwall projector, a $5 mini disco ball unit from Walgreens, and a Madame Leota loop. The disco ball rotates, but I only did a stationary test:

 

I like the mirror ball idea. That looked cool.

 

Glad you enjoyed it. I thought it was cool too when I tried it out. Will also second what Icepick said about reducing throw distance to your projection surface if you're in a tight space. May experiment with this myself, so I'm enjoying reading this thread. Can also validate what Creepy Creations said about using a mirror; at least to bounce the projection to a space you may not be able to reach otherwise, like a high window on a cathedral ceiling.