My significant other (23 at the time, otherwise perfectly healthy) felt a sudden headache one evening. Since it didn't stop, she was taken to the emergency room immediately, where they've dismissed her with "menstrual problems".

A day after that the pain didn't stop, she ended up in the hospital, and had a (fortunately successful) brain operation some days later. Luckily it was in an accessible place, so she ended up with no apparent long-term damage.

Definitely a terrible situation to be in. About 4% of people have it, and a small percentage of those only find out when it bursts.