When I'm asleep, I'm really susceptible to believing I've killed an animal that is resting beside me. There have been multiple times when I have thought I killed my girlfriend's cat because she was lying perfectly still next to me when I woke up in the middle of the night. This feeling is compounded when the animal is a deep sleeper and takes awhile to rouse.
Last night my family's new puppy came and slept with me before my mom had a chance to put her in her cage. I drifted in and out of sleep for a little before I finally woke up for a bit and felt the dog down at my feet. I reached down to pet her to make sure she was okay, and she was just lying there stiff as a board. This began freaking me out, as repeatedly poking her and trying to wake her did nothing. At that moment, I was convinced I somehow killed the dog for the brief period I was asleep. I even picked her up and moved her, and her body was so limp that I was still convinced she was dead. Eventually my mom came and got her and she moved around a bit, so I was relieved to see she wasn't dead.
Still, having an animal sleep in my bed has become one of the most terrifying things in the world to me.