1. You expect me to believe that you don’t know how that dent in the car fender got there? Oh, come off it!
2. First you ask for juice and then change your mind and say you want milk. I get it for you, and now you beg for water. Come off it!

The expression is always used as an expletive in the command form. It is very informal and would normally only be used by parents with their children, or between equals.