PC used for CCTV in a Chinese Takeaway. PC sat on a shelf in the kitchen. Open that one up - everything covered with a thick layer of grease. Not ideal for keeping fans spinning.
Another job, another case. Large barn where they made paving slabs, kerbstones, etc. Concrete dust was an inch thick on every surface in the PC. Hate to think what state the hard disk was in as that dust would have got through the breath hole.
Do not combine fluffy cats with wooden floors. A Dell sitting on the floor is an excellent hoover. Pull the front off of that case and the hair was so thick and jammed that it came out as one solid lump.
Never assume IT knowledge. One person I knew would store important documents in a special folder on the desktop. You know the one. It has a special icon and sits in the corner. When fixing her PC it was lucky that I checked with her before I emptied the recycle bin! I don't blame her, no one had taught her to use a PC.
Another bizarre thing I learnt from her was the total lack of understanding how one window could sit on top of another window. No concept of the 3D way the GUI would stack windows on top of each other. Meaning she would only run one program at a time and get totally confused if a window disappeared.
I'll censor some of the rest... like the house of the mad cat woman with a least a dozen cats. Dried cat sick down the side of the case. I learnt to carry gloves after that visit...