I've seen-em too
A friend with 3 dogs and the PC on the floor asked me to do some upgrades - the dust-bunny was half dog-hair and completely filled the open space in the case. I have no idea how it kept running.
And way back, in the Apple /// days, I worked on one and really did find a dead mouse. Little turds and pee-stains all over the place, but I guess one day he hiked a leg over the wrong component and ZAP - he died along with the 'puter.