Discomfort. You should come down from Sirsasana (Headstand) or Sarvangasana (Shoulderstand) if there is pressure or discomfort in your head, eyes, ears, neck, or throat.
Cervical spondylitis. Characterized by reduced disc space between the cervical vertebrae, bony spur formations, a loss of the lordotic (normal) curve of the cervical spine, and numbness and tingling in the hands or feet due to nerve compression.
Cardiovascular disease, hypertension, and/or excess weight. According to Ralph Laforge, static inverted poses can pose problems for those with coronary disease because they "place undue pressure on the chambers and valves of the heart, which in turn places undue stress on the already compromised coronary circulation." If you have high blood pressure, the baroreceptor mechanism which protects the brain from an excessive surge of blood may possibly be compromised in inversions.
Diabetes and/or insulin resistance. According to Laforge, virtually every adult with type II diabetes has coronary disease, or "silent ischemia." The internal pressure on the arterial walls from blood volume changes can result in cardiovascular complications.
Glaucoma, detachment of the retina, or extreme nearsightedness (retinal detachment is more common if one is extremely nearsighted). Unlike the brain, the retinal veins of the eyes might not be protected from a sudden onrush of blood by inverting. If your eyes become repeatedly bloodshot from inverting, you should consult a teacher or medical professional.
Discharge or infections of the ear, sinus infections, and the flu (although inversions are recommended to hasten recovery from the common cold).
Pregnancy past the first trimester, although this can vary greatly depending on your familiarity and ease with inversions. Some women find that supported Shoulderstand with a chair or Viparita Karani can be of benefit well into pregnancy.