I myself have never had a really bad delusion, but at the same time, I become super cautious on acid. Other people, though, have gotten kind of weird on acid.
One time, after having watched A Scanner Darkly on my first acid trip, I went to my friend's room (we live in a dorm) and he had the lights totally out. I wasn't afraid or anything at first, but I felt like the dark was suffocating and that any one of them could inadvertently lash out at me, or worse, there was someone else in the room I didn't know about. I was borderline ready to freak out at them when I told them to turn on the light, and they did, thankfully.
Later that day, I go back to the room, and they start laughing as soon as I walk in, telling me I had to leave because two men in black suits had come there looking for me. Of course, I think they either hallucinated it or were bullshitting me in order to fuck with me (I'm assuming the latter to be true) so I tried to reason with them, telling them that if anyone were looking for me, they'd be going to my room first, and I definitely would have already been found. WELL, they weren't having that so I decided to leave them and just go chill with some hippie kid from a few floors up.
This other time, our provider of drugs came to the house while me and my fellow pledge brothers were forced to clean the whole place while high off a grav hit. Almost everyone took some acid, including this one kid who'd never done it before. And he freaked the FUCK out.
First, he believed that the guy who'd given him the acid was God, and started praying in his native Bangladesh tongue. Then, he thought the guy was a Satan figure, so he started freaking out and trying to attack this one girl that was there. We told him to stop, and that it isn't cool to attack girls. So he says this:
"No girls? No girls? I don't like girls! I like boys! I'm gay!"
(I'm thinking this was just his line of reasoning, since we told him not to touch the girl he was trying to rape, that he must not like girls, which made him think he was gay.)
Then, he whips his dick out and starts flopping around on the floor, once again yelling out prayers in his native tongue. The guy who gave us all the acid had to drag the kid off, get his pants back on, sit him down, and get him to relax. Me and one of my other pledge brothers (who was not tripping, just high) decided to sneak off to China Wok and have some food, then relax and try to get past what we just witnessed at the house.
On our way back to the dorm, I was convinced that I was walking through a snow tunnel, even though the snow was only about a foot high. And my friend talked to me as The Snow, which had the voice of a black man for some reason. Ever since then, he assumes this voice that talks to me as various objects, usually any time I'm tripping or just high.
Another time, after my ski tripping experience, I sat out in my backyard. The sky was totally cloudy which bummed me out, and I thought: I want it to clear up. Maybe if I sit here it'll clear. What freaked me out was that it did. I had to tell myself: this is what you wanted, now let's just enjoy the show and relax. So then I thought I could see/hear the stars jamming out to Chuck Mangione's "Feels So Good," (no idea why it was that song, but it was cool because my mind would also improvise solos with all kinds of strange instruments) but that got disrupted by any airplane that flew by. I saw a couple of cool cloud-shapes too, the most vivid one being a scorpion trying to grab the moon with its claws.