A cyst may be caused by a defect during fetal development, an infection or some unknown reason. Many people may notice small, hard lumps under their forearms or wrists, for example. These lumps are actually a form of cyst called a sebaceous cyst. A lump in the wrist may be a ganglion cyst. Some people develop a cyst on their vocal cords. It is not unusual to discover a cyst under one's scalp or face.