Many people experience nausea and/or vomiting during mushroom experiences, especially with higher doses. This is largely due to the somewhat indigestible nature of the fungus coupled with the mental reaction to the active ingredients. Generally the nausea is mild and passes shortly into the experience.
The nausea can also be quite strong, persistant and annoying, even from a medium dose. Usually, the best course of action is to give into it and throw up. Fighting it for long periods of time is difficult and most often futile. Although 70% of the nausea may be psychological, the other 30% is real and keeps it on your mind no matter how hard you fight it.
In this writer's experience, mixing 2 tablespoons of ground ginger with the powdered dose of shrooms has been 60-70% successful in preventing the nausea altogether.
Other possible negative effects include anxiety and unwanted or frightening thoughts and visions. Mushrooms, though perhaps to a lesser degree than LSD, can precipitate strong, temporary changes in an individual's experience of life and reality. It can be a powerful psychoactive experience, especially at higher doses, which is significantly affected by experiences, set and setting.
Recent experiences, especially strong ones, can have a substantial effect on a trip. Physically or psychologically unsettling events in the days before a Mushroom trip can blossom into more serious distress and trauma while tripping. It is important to be prepared for the possibility of encountering difficult or frightening mental states. The Erowid Psychedelic Crisis FAQ addresses some of the issues involved in dealing with a difficult trip.
Vomiting is a mechanism to expell toxins from your body and prevent them from entering your brain. In this case the 'toxin' is psilocybin.
"Vomiting is co-ordinated in the vomiting center in the medulla. Receptors on the floor of the fourth ventricle of the brain represent a chemoreceptor trigger zone, stimulation of which can lead to vomiting. The chemoreceptor zone lies outside the blood-brain barrier, and can therefore be stimulated by blood-borne drugs which can stimulate vomiting, or inhibit it."
Some people find that with vomiting, the visuals (both OEV and CEV) asser themselves more readily and intensely. This is purely subjective (as with most of the rest of the experience ;)