I do believe I have hallucinated mildly off weed. I was about 15 beers down at a house party, so I can't rule out any interaction effect however. I took two massive hits off a fat dutch tulip someone had brought (my tolerance is pretty low, thought that would be fine though). Felt ok initially, just like the beginnings of a high. About 5 minutes later, I was rushing up. Suddenly, everything went yellow, kind of like the sepia tone you see in movies. Then, my entire field of vision was filled with a rotating, fractal like mosaic. Looked kinda like floating crystals. I was totally unable to move, and could do nothing but stare at the floor (which was overlaid with this pattern). I remember thinking "Holy shit I'm hallucinating at the moment"...after that I lost all motor control and passed out. Came to about 10 minutes later and was fine, just super high.
Might have been cannabis-induced hallucinations, might have been oxygen deprivation. If you're taking massive hits and holding them for ages, you're probably getting pretty mild hypoxia (symptoms of which can include confusion and hallucinations). But I think THC hallucinations are possible. Getting high for me means I can "see" my thoughts and day-dream images a lot more clearly (eyes closed). I can essentially go on little dream-like adventures in my head, and they feel a whole lot more immediate and "there" than in when normal. So get enough THC into you quickly enough and perhaps that can happen open eye?!?