Sometimes very literally! Flying SJC-IAH a few weeks back we had a training crew on board. The lady that introduced herself as the purser, and made all of the standard announcements, was a trainee FA who who was probably in her early 20's, and who (based on what we were told on the flight) was still a few flights off graduating as an FA. The nerves and inexperience were clearly showing as she did the announcements, but she did a great job regardless!

In fairness, after she'd done all the initial announcements a second FA introduced herself as the "2nd purser" and explained about the trainee crew (they had an additional FA on board for each of the trainees, although they did spend most of the flight doing nothing which I'm guessing was the intent).