I once saw something that I couldn't believe.
I was at home and in my dining room, through an open doorway I have full view of the kitchen stove. I was sitting down and out of the corner of my eye I saw something move in the kitchen. It was evening, but the lights were on, I was just home from work. What I saw shocked me because of it's impossibility, as I stared at it, I saw a long thin "tentacle" protruding from a vent underneath the controls above the stove-top surface. It was brown-ish, and slowly moving, like a finger gesturing or feeling around, and appeared as if it were 5-6" in length, about as thin as a pencil. My mind couldn't comprehend what I was looking at, why it was there, what it was doing inside the stove, it was horrifying, yet I was frozen and could not look away. After what I would guess was 15-20 seconds it silently slipped away. Afterward I was able to get up and take a look around, but did not find anything with my flashlight inside the oven or underneath where I could look.
I still don't know what I saw, it was either a hallucination from being tired after work or a really big mouse inside the stove. I never saw it again in that house while I lived there.