Lots of things can go wrong. The most common are trailed stars, computer problems, and bad focus. This image of The Crescent Nebulas, has a lot of them. I started out the night with a computer problem, requiring about 4 re-boots. Then it was a problem with a guider trailing stars in the wrong direction. After I thought it was all squared away, I downloaded the night's images, and found the focus was slightly off.
Some nights you just can't win. Here is the result of this problem ridden imaging debacle. To this date, I have never taken a good image of the Crescent Nebula, NGC 6888.
This is a stack of 20 images at 8 minutes each through my C9.25.