I've seen spiderman on my roof.
I've thought the car i was sitting in was a submarine
I thought my sister's arms were made of hoses

It was just an interpretation of shadows and silhouettes. Or an unexplainable feeling that these things were plausible (geez, i can't explain the origin of them!). A few of my friends say they've never hallucinated, but between my personal experience and the other posts on here I'd say there's no doubt that it happens.

I usually hallucinate only while smoking alone, when my mind is free to roam and i don't have to worry about passing something, or getting caught up in a laugh orgy.