Yes, poor performance could be one. Breach of contract is another, for example, doing something counter to your job. This is different from gross misconduct, as gross misconduct doesn't have to be enumerated in your contract. For example, punching your colleague is both illegal and will likely get you fired for gross misconduct. My contract doesn't have a clause that says "don't assault your colleagues."

Yep. But generally not instant dismissal.

Although apparently they did actually try to pay her for her notice period and she sent it back? This case is a whole load of fun to read through.