The session management mechanism provides a rich source of potential vulnerabilities for you to target when formulating your attack against an application. Because of its fundamental role in enabling the application to identify the same user across multiple requests, a broken session management function usually provides the keys to the kingdom. Jumping into other users' sessions is good. Hijacking an administrator's session is even better; typically this enables you to compromise the entire application.
You can expect to encounter a wide range of defects in real-world session management functionality. When bespoke mechanisms are employed, the possible weaknesses and avenues of attack may appear to be endless. The most important lesson to draw from this topic is to be patient and determined. Quite a few session management mechanisms that appear to be robust on first inspection can be found wanting when analyzed closely. Deciphering the method an application uses to generate its sequence of seemingly random tokens may take time and ingenuity. But given the reward, this is usually an investment well worth making.