The hellmouth has had a strange effect on Sunnydale and the people who live around it. It has a presence about it, that even when unopened it can spread it's evil far and wide. The local high school being situated on top of it meant that the students of Sunnydale High experienced the worst of strange happenings whilst attending. It had a tendency to make what ever the teenagers were feeling become reality. For instance one boy felt like his head was going to explode through the stress of exams and it did! Here are a few other cases of weirdness in Sunnydale that have been caused the effect of the hellmouth alone.
Billy Palmer was a young boy in a coma in the local hospital after being battered half to death by his kiddy league coach. Who somehow unknowingly managed to bring the nightmare world that he was trapped in into this reality. Everybody's worst nightmare came to be and Sunnydale was in chaos. The only way to rid this reality of the nightmare world was to wake Billy up, which isn't as simple as it sounds as Billy was not just asleep but in a coma. So instead Buffy helped Billy to face his nightmare and then he came out of his coma and Sunnydale returned to normal.
Marcy Ross was a very lonely girl because she was completely unnoticed by her fellow students, she had no friends and nobody even knew her name. Reality is shaped by our perceptions of it so because people perceived marcy as invisible she then became so. The loneliness and constant exile eventually sent her mad, so she lashed out at the one person that had everything she did not, Cordelia Chase. She was prevented from hurting anyone by buffy and then taken by the government to be trained as an assassin.