Oliver Sacks' book, Hallucinations was a big eye-opener for me. He describes just how common hallucinations are.

You can be perfectly sane and experience hallucinations. Hallucinations are common among people who are losing their vision. Sacks even describes some of the hallucinations he himself has experienced over the years.

Sacks tells an amusing story of someone who was aware that he was hallucinating because of some medical condition. He saw someone waving to him from outside a window while he was inside a tall building, and assumed it wasn't real, and so ignored it. It turned out it was a window washer who was perturbed he didn't wave back, lol.

Even the visual migraines that some people experience are technically considered hallucinations.