Hmmm, I've had A LOT of different hallucinations whilst rolling. These are my most memorable ones:
Whilst walking through Bradford City Centre with one of my mates, we walked past a large billboard with 2 people hugging, and I watched the couple lean in close to kiss. That was my first hallucination, IIRC.
That same night, I kept looking at my boots, convinced that they were my dog.
My most memorable CEV was one of a newspaper, with a headline asking me if I was ready to go into space. I was trying to get some sleep at this point, and I said out loud, No! I'm trying to sleep! My girlfriend woke me, asked me what the hell I was talking about, and I just burst out laughing.
Another night, I was in a club with some mates, and one of them and myself decided we'd do some lame Mortal Kombat pisstake, just doing lame fighting stances, when I watched him turn blue and begin to take the form of a rather ominous looking demon figure. He was also rolling balls, and at the same time, he watched me turn into this strange pinkish monster with pulsating red veins all over. We just stood there, tranced out for a couple of seconds as this happened. We snapped out of it at exactly the same time, looked puzzled at each other for a second and both said "whoa. What the HELL was that?"