This won't be a contender so far as horror stories go, but: a couple of years ago I started noticing that I would be watching TV, or sitting in the living room, and would hear a little "skittering" sound in the fireplace area. I looked around in there with a flashlight different times when it happened, but everything always went dead still then, and I could never see a thing. I pretty much thought in terms of a mouse. Then one night all "heck" broke loose, with major clattor, spitting, hissing and snarling. I opened the flu a crack, and there was the "bandit mask" glaring and threatening evil. It was some late hour; I wasn't about to try to deal with it that night. I figured the darn thing was trapped down there, and we would have a pain in the rump getting it out the next day. But apparently it could, and did, climb back out. I got a screen cap on the chimney at first opportunity.