(1) No film in the camera. Now this is dumb and why is it even mentioned? Smart astrophotographers have and use a "checklist" of things to do and equipment to take along on their outings. Most are very meticulous that everything they need to do or take along is taken care of. So much attention is paid to small details that occasionally the obvious is overlooked. Don't laugh, I've done this one myself. Imagine sitting at a guiding eyepiece for a couple of hours and getting that "perfectly guided shot". Now it's time to advance the film a couple of frames and start another photo. You cycle the frame advance lever and suddenly realize that it is moving a "little" too easy. DUMB, DUMB, DUMB, @%$&!#@%!!
(2) Shutter not set to "B" setting for guided prime focus shots. I've done this one myself and even made 3 or 4 guided shots before the error was discovered. From then on, I make it a point to double check the shutter setting before starting the shot and even to "listen" to the camera when the remote cable button is depressed.
(3) Left something at home. If you have to drive several miles to a dark site, then prepare a checklist of items to take along and religiously use this checklist before leaving home. Never trust your memory because, after all, we are human. I once drove about 50 miles on a perfect night to a dark site. After setting up the tripod and attaching the wedge, it was now time to mount the scope. Now where are those #$%#$% three mounting bolts to fasten the scope to the wedge?
(4)Dangling Wires. Make sure that any wires hanging from the scope are out of harms way and won't be inadvertantly snagged while working around the scope at night. I usually take along a few rubber bands to tie loose wires up and out of the way. If the scope is powered from your car's cigarette lighter, a piece of plywood layed on top of the 12V power cord going to the scope will prevent you or someone with you from tripping over the wire and disconnecting it from the scope, or even worse, breaking the wire and ruining a whole night. When tying up wires, make sure that they don't interfere with the tracking of the scope.
(5)Tripod moves while making photo. This pitfall is commonly encountered when the scope is set up on soft ground due to a recent rain. A heavy scope used on an equatorial wedge places a large "off-center" load on the tripod. If the ground is not stable, over time, the tripod leg bearing the most weight will settle into the ground slightly and throw off the polar alignment. If these conditions are expected, it is a good idea to take along "footing" pads for the tripod. These can be nothing but simple wooden boards to spread out the load over the ground.
(6)Polar alignment not properly done. When using the scope for observing purposes, an accurate polar alignment is not called for, and in reality, it is foolish to waste valuable observing time trying to get a perfect polar alignment. Astrophotography, however, DEMANDS a very accurate polar alignment especially if long exposures of 15 minutes or more are anticipated. Time spent here is well used since a "so-so" alignment will result in "so-so" photos. Learn how to do the drift method of polar alignment and how to do it well.
(7)Problems with dew. If you live in an area of the country where dew is common, then by all means get some form of protection from dew. I've had this gremlin bite me several times while in the process of making a photo. This problem is not too great with reflector telescopes but with refractors, Schmidt Cassegrains or Maks, it is a major concern. These latter scopes don't have the benefit of a long tube to cover the optics such as Newtonian reflectors do but rather have the objective lens or corrector lens at the very front of the optical tube where they are exposed to the night air. Glass and metal surfaces radiate off their heat much qucker than fiberous or wood objects and in doing so, cause the formation of dew.
Devices to protect the optics from dew formation include "heaters" that reduce the heat loss of the objective lens or corrector plate and thus keep its temperature above the dew point temperature. The Kendrick dew removal system or the Orion Dew Zapper are popularly used items that work quite well. Another effective protection measure is to use a dew shield over the front of the scope to protect the lens or corrector from direct exposure to the night air. Another big plus for using a dew shield is that stray light from vairious sources will have a harder time getting into the scope. Personally, I prefer to use a heater AND a dew shield.
(8)Not preparing for the elements. Ever notice that it tends to get colder at night or that insects do most of their biting after dark? Long periods of inactivity while hunched over a guiding eyepiece don't do much for keeping the blood circulating and the body warm. Effective guiding simply cannot be done if you are freezing to death or if during the warmer times, the mosquitos are having a banquet on your body's valuable fluids. Manual guiding takes 100% of your concentration and you simply cannot be distracted by being uncomfortable. So, add to your list of things to take along items of warm clothing during cold weather, insect repellant for warmer times and yes, snacks or beverages. Now some folks will say, why not use an autoguider and stay out of the elements? The answer is boredom. Sitting in your car for long periods of time doing nothing is time wasted. While the autoguider is doing its thing, why not use the binoculars or even that second scope if you have one to enjoy the night sky.
(9)Part of your equipment gets broken or doesn't work . It's always a good idea to take along pieces of equipment or hardware that could salvage an otherwise ruined trip. A second remote shutter cable is high on the list since the little locking thumbscrews are notorious about stripping the threads. Additionally, spare screws, bolts and a set of tools to do minor repairs to your equipment are good items to have on hand. An extra 12V extension cord is another item that is good to take along. If you use a seperate battery to power the scope, guiding eyepiece, or other equipment your have, make sure it is charged or fresh replacements are on hand. Don't forget flashlight batterys.
(10)Photographing on bad nights. Now this is the most common screwup that everyone makes, including me, even now. Excellent astrophotos cannot be made unless the seeing is exceptional. I use three checks to determine seeing conditions. The first thing I make sure of is to not photograph when the moon is out (unless is is early waxing or late waning), the second is I look at the Milky Way and if it is bright and extending nearly horizon to horizon, I know the sky is very transparent. Third, and last, if the first two checks pass, I then look at the stars. They should be steady and not twinkling. Twinkling stars are pretty to look at but they are a sure indication that the atmosphere is unstable. Photography under these conditilns will result in photos that seem poorly focused due to the scintillation (or apparent movement) of the object or stars due to refractive effects of the unstable air.
(11)Record keeping. Always keep a log on what you have done. As mentioned above, never rely on memory as it invariably will fail you. Having a database on what was or was not successful in the past will do wonders for your chances of getting good astrophotos in the future. Why repeat previous mistakes when they are so easily avoided? This is a problem I still have but, thankfully, I am getting better with my record keeping and my success rate is improving.
(12)Don't sit at home. Astrophotography is a hobby that is to be enjoyed. You can't become proficient at it unless adequate time is devoted to it. Keep an eye to the sky and when conditions are right, jump at the chance, get out the scope and neccessary equipment and go to that dark site. Unless you live in the dry stable climates of the southwest, the opportunities for making photos on those rare nights when conditions are right occur too infrequently.