What will get an employee immediately fired?

I've worked in many different environments (sales, finance, banking, training). I have also been fired from a job before (not immediately, but after several warnings) for tardiness. I would say a good golden rule to live by is "always be honest". Integrity will always guide you in the right direction- to do the right thing. Different companies may fire for different reasons, it all depends on their policies. As stated in other answers you got on your question generally actions of dishonesty (stealing, using company resources wrongfully, lying to customers, abusing power, etc) can cause someone to get into deep trouble fast. No matter where you work a great way to stay out of trouble is to stay informed and updates, and ask questions. If you don't know whether something could get you in trouble, ask someone before doing it (manager, peers) or read up on their policies.

Lack of integrity, fraud and deceit. For example, committing crimes like financial fraud, misleading employers with false educational and professional qualifications, and lying in general. You will get fired immediately, period. So don’t go there, ever. Be truthful and honest, authenticity is the cornerstone to life and career success, no matter what your definition of success is.

Policies and procedures are put into place for a reason as well as the mission and code of conduct. When a employee doesn't follow these that puts them more at risk for immediate termination.

In my experience as an HR Business Partner an "immediate" termination is rare.  Most businesses will do an investigation into the alleged activity and allow the employee to state their side of the story prior to separating them.  Acts that would warrant an investigation would include violating policies or the Code of Business Conduct.  Examples would include theft, violations of the substance abuse policy, sexual harassment, lying about sales results or expenses, etc.  The bottom line is this:  The only thing in your career that is100% in your control is your integrity.  Don't ever compromise it for anything and you will not be in a position to be immediately terminated.  Make sure you understand your employer's policies and if you have any doubt if an action will be problematic, ask your supervisor or an HR Manager.

I would say in some cases, an employee's behavior may merit immediate dismissal. The employees behavior could be committed in front of co-workers or in front of a customer. HR will typically will investigate the matter prior to making decision, is what I have seen take place in the past. The employee may simply have knack for saying something inappropriate and creates a unwelcoming work environment. This can put a business at risk for a sexual harassment claim. It's important to do something about the employee as soon as possible, taking action is key.