I've read a lot about 'thought loops' and what was described by user Changed, is definitely the sort of thing I've seen/heard others act when they've gone a bit loopy on their trip.
Me, though, I've yet to experience this. On LSD I can rarely remember where I am or what I'm doing, let alone repeating sentences or even speaking in a coherent manner is largely impossible for me.
Are these thought loops characteristic of negative experiences?
My last two psychedelic experiences have spiraled into thought loops. First was on my largest dose of mescaline, and the later was on a fairly large dose of mushrooms.
With the mescaline, my trip setting wasn't ideal: first I went outside with my friend in the freezing cold and ate some pancakes. We talked normally (actually, even more philosophically than we usually do), then came back and I listened to Pink Floyd in the dark. I quickly was jettisoned into oblivion, where I had instant knowledge of every event ever occurring in and outside of our universe. My thought loop simply went from a small piece of knowledge, all the way up to the greatest possible implication. The trip report (which, I must say, is an interesting read

):
http://www.bluelight.ru/vb/showthread.php?t=473844&highlight=substancecode_mescaline
In retrospect, it was a pretty cool thought loop to be stuck in, compared to the following mushroom trip...
My friend, totally inexperienced with psychedelics, decided he would do mushrooms with myself and two other recent acquaintances. We procured and ingested the mushrooms while fully drunk and at 3am. Terrible idea. We both lay down and listen to Boards of Canada while dinosaurs and eagles and whales fly out of the speakers. Trips going fine until he disappears. Of course, he had simply gotten up, walked out of the room, and gone to bed without telling me. I simply cannot perceive being alone in the room with the music. I get up, stumble to the other room and attempt to fall asleep on the futon. I last 5 seconds and I'm up pacing the apartment. Where is he? I'm thirsty. Is the gas stove on? Is my friend alive? Is the gas stove on? I'm thirsty. Okay, I drank water, did I drink too much? Am I going to drown? I'm still thirsty, I can't have drank that much. Where is my friend? Why is everything watercolor? Why can't I read? Is the oven on?
I literally walked around the apartment for 2 hours in a circle making sure everything was okay. I finally got into bed, still tripping balls, and begin another loop: "Why do I feel like this? I'm tripping. Okay, I'm just tripping. Oh god, everything is a watercolor painting!! WHY DO I FEEL LIKE THIS! I'm tripping. Okay, I'm just tripping. Oh god, am I dead? I'm dead. No, I'm just tripping. Okay, I'm tripping. Oh god, is this the afterlife?"
This went on until I finally passed out. I was literally like a goldfish: my memory lasted (what seemed like) 3 seconds, and I'd revert back to mind-less mode. I knew I had to tell myself that I was tripping, but I simply forgot 3 seconds later.
Now that I think about it, these two experiences may deter me in the future from partaking in any further psychedelic experiences.