What are some mistakes you can make as a programmer that will get you fired immediately?
Taking one firm’s proprietary information to another will do it.
Pepsi does not want the secret formula for Coke, and AMD does not want Intel design data, to pick two publicized examples. If you merely get walked out on the spot, you are lucky; criminal charges are possible.
An example that’s not so widely known: I know a developer who worked at a computer company, where he wrote internal software that was not supposed to leave the company. His software would periodically report its status by sending an email to a server that collected status data. Both the software sending the emails and the server receiving the emails were supposed to be on the corporate intranet.