What is the worst that could happen if you pierce your own eyebrow?

Nerve damage, excessive bleeding due to hitting a vein, blood infection, blood poisoning, Staph infection......... I could go on but I think you get the point here, the human body is not a pin cushion, however when you get a piercing from a professional body piercer you will have a piercing that will heal and look good for a long time without the grief of botching your eyebrow piercing and doing a whole lot more damage than you were counting on. Leave the piercing to the professionals.