Definitely! ...and, some people are crazy! I've worked with schizophrenic patients and people with psychotic disorders- there's plenty of them running around too. Also, hallucinogens most definitely factor into this thing too. Again, not directing that at anyone in particular.
Mostly though, I think Yarak is right- people claim some goofy stuff sometimes when their imagination gets the better of them. I think I'm at least half sane and I've sworn I've seen things that my buddies have told me simply weren't as I saw them. I swear I saw a Irish setter one time on a deer drive, but both my brother and cousin saw it and they both informed me of the fact that it was a small buck. But dang I was sure! However, there's no doubt I was totally wrong. Just one small example of how things aren't always as they appear...