Main shaft may crack or break. A loose strand of hair may also be wound up in the main shaft. This causes the main rotor to slow down, and may even stop the rotor from spinning entirely, if serious enough. Do not fly with a cracked main shaft. If this happens, the best thing to do is remove the shaft from its holder and replace it (or just remove the hair, if that is all). Usually, this entails removing the main gear and any securing screws, so make sure you remove all that hold the shaft in place and place them in a secure location! These are the most important screws on your RC helicopter.
Take care to avoid removing any extra grease, as this will make the shaft a bit less efficient in rotation. For a coaxial, in order to remove the outer shaft you must first take out the inner shaft. In the picture to the right, the main gears are in the bottom white box, and the inner shaft is denoted by the top white box. The inner shaft always connects to the lower gear, and the outer shaft to the higher gear. So to remove the inner shaft, be sure to remove the lower gear. If you must remove the outer shaft as well, you will likely have to remove some extra screws. For a general idea of how to remove the inner shaft, view the first bit of the lower blade swap video on Volitation Mods and Upgrades.
For a CP / FP Heli and 4 channel coaxials (and some 3 channel ones), you will have to disconnect the swashplate from the servos. Simply pop off the pushrods (or rods with circular openings at the top) from the swashplate. Then remove the main gear and any necessary screws (which there may not be any). You will be able to tell when the shaft can come out because it will slide out of the frame with just a push. It should not require much effort, otherwise there is still something holding the shaft to the frame.