Occasionally, you'll find an anime that goes beyond the manga. This can happen if the creators decide to develop new storylines or flesh out certain aspects that weren't as detailed in the manga.
Yes, there have been cases. Sometimes an anime might expand on the story or add filler episodes to prolong the series.
Sure, it's not uncommon. Sometimes the popularity of a series leads to the anime creators coming up with additional content that extends beyond what was originally in the manga.