Great Book of Errors
Don't hang your head - you won't see the stars
Stars show small jagged spikes at the outline. The stars may be round or elongated.
In most times wind causes the scope to judder. Maybe the mount is overloaded. The bigger the load the sooner it starts to oscillate. In some cases the shutter of a DSLR Camera can be enough to cause the vibration.
Avoid windy places and vibrations. Reduce the load or get a stronger mount. Always use a remote to release the shutter. Some DSLRs have a pre-release of the mirror. Use it if possible.
Sometime you get a similar result if the auto guider works to aggressive and overcompensates a small drift.
'Roughly' Star trails .
The Guiding was not precise enough. Either the autoguider was not working or you attempted to take pictures without guiding. These little spikes along the trail occur due to mechanical imperfections in the mount's gears.
Switch on autoguiding ;)
Steady drift of stars during exposure and in between. This may be along one axis or diagonal. The length of the trails is proportional to the exposure time. It may differ depending on the place the telescope is pointing at. Note that all shift is parallel.
This took me 10 nights to figure out - even the cause is quite simple. The mount's motor is not capable of keeping track during autoguiding. The mount is overloaded or badly balanced. The reason why the error may differ depending on the position the scope is looking at is that the unbalance might be worse in certain positions. As the telescope's mass is not uniform (camera, viewfinder, guiding scope...) it is impossible to balance it perfectly as you spin it along two axes.
Solution: Re-balance it right before taking the picture, make the scope as light as possible or get a stronger mount. Check your power supply. Low voltage can be critical.
Stars are differently elongated in different part of the picture. They spin around a centre which may be inside the picture or outside. This error occurs even when autoguided.
The mount is not correctly aligned. The RA-axis of a equatorial mount must point precisely to the north (or south) pole of the sky. Otherwise it will spin around your guiding star.
There are several ways to align the mount. For short exposures it is enough to use the polar scope - if the mount has one. For longer exposures you can align the mount according to the 'Scheiner' method. This is very precise but takes at least one hour. Some electronic controllers (like synscan) have electronic alignment algorithms which are very handy. Check if your tripod is still level.
Every star appears twice. The offset is homogeneous across the image.
Typical appearance. Many possibilities may underlie. The guider lost the guiding star, searched for it and discovered it later again with an offset. In most cases mechanical imperfections causes this kind of error.
Check the steadyness of the guiding scope's mounting plate. Re-check the clamping. Look out for any instable part at you equipment. Mirror shifting might be another cause for this kind of error