I was taking a panorama of Cassiopeia. That panorama didn't work (not enough overlapping), but there was this "comet" on one of the images. I immediately checked the other images as well and realized what this was. There's a star in a very convenient place to make the flare look like comet's tail. The next image had the same flare but the star had moved away from it.
Identical Ghost Nebulae
I was testing whether I could photograph the Horsehead Nebula. I had the EQ-mount surprisingly well set. I could take 30 second exposures with 200mm, where I usually have to settle to 20 sec or less. Flare nebula was easily visible on camera screen so I started to take series of photos. Then I noticed this:
First of all, there shouldn't be anything like that on sky, at least on Orion. But now there's two identical! I was taking the photos on my balcony and there were some lights from neighbors that might find their way to my camera even though I had quite a long hood on the lens. I put some cardboard to block all light I could think of, but still the flares remained.
I zoomed and looked the image thoroughly and I saw a third one. Then I got it. Those flares weren't from lights around me but from the stars. The next image (click it larger if the small one doesn't show) explains it quite well. There are three flares and three bright stars. Moreover, the positions of the flares are exact π rotation of star positions.
The flares were caused by UV-filter I had on the lens. I read somewhere that the filter does not affect anyhow on stellar photography so I've kept it on the lens to cover it. Looks like I have to remove it from now on. Without it there were no flares.