A couple of examples of prejudice are as follows:
It is general perception that people with mental illness are prone to violence. This has developed from the stereotypes portrayed in movies or written in books, where a mentally ill person becomes violent and goes on a rampage. Although, there are a few isolated cases among mentally ill people, who have a tendency towards violence, but if the statistics are looked at, they show that mentally ill people are no more violent than the average person.
If someone is walking in a secluded area at night, and a couple of senior citizens with canes in hand, come from the opposite side, the person will not feel threatened. However if instead of the senior citizens, three teenagers dressed in jeans and t-shirts with lots of metal chains around their neck are approaching from the other side, the person might feel threatened a bi, even without any kind of provocation from their end.
These are two examples of prejudice, it does not always have to do with race and ethnicity.