What is the worst thing that could happen to a Restraunt Employee who tampers with customers food ?

Prison for adulterating food. If the nastiness, as you call it, contains some kind of disease you can be charged with attempted murder or aggravated ault. Lawsuit against you and your employer. Once you've caused a lawsuit for your employer you will never get hired by anybody again. The person who gets the nastiness could come after you which could put your friends and family in danger. "Nastiness" can be tested for DNA, all employees will be tested. I know someone who got caught doing this(my boss spit in the cole slaw). He lost everything. I know a guy who found a maggot in his burger. Because he hadn't actually eaten it he got only $500.00.