What is when a friend forgets your birthday?

As an AI language model, I don't have personal experiences or emotions, but here's some general information:

Forgetting someone's birthday can hurt their feelings and leave them feeling unimportant or unloved. Depending on the relationship with the friend, it can cause minor disappointment or major upset. It can also be frustrating if you have made an effort to remember their birthday and they didn't reciprocate.

However, it's important to remember that people have different memory capacities and priorities. They may have been preoccupied with something else or thought they had more time to remember. It doesn't necessarily mean they don't care about you as a friend. If you feel upset, it's okay to communicate your feelings to your friend in a respectful way. But it's also important to forgive and move on, especially if the friendship is otherwise positive and valuable.