Over the years I've found rocks, marbles and even BB's in fuel and gas tanks on tractors, machines and even trucks. Had a thread on here before about someone before I owned the dozer putting road tar in the fuel tank, worst mess ever.
Years ago my father in law forgot to put the fuel cap on his four wheel drive tractor one night while doing tillage and ran all night with no fuel cap on, the tractor quit before morning and the fuel filter was plugged with corn stalk debris, mud and sticks, the tanks had to be removed and dumped out, washed out and put back on again, but even years later corn stalk debris would show up in the filters every now and again.
I've seen a combine fuel tank that had shelled corn in it so bad, no fuel could get out of the tank, corn swells and over time it even puffed out the tank somewhat, turns out over winter the tank cap wasn't on tight and mice had knocked it off and filled the fuel tank with shelled corn off the machine.
A neighbor had his combine in the shed over winter and come spring, the engine wouldn't turn over, after working for day's, he gave up and called the dealership to come out to fix it, turns out mice had crawled down the exhaust pupe and filled the engine cylinder full that had the valves open and the one cylinder was full of corn and the engine could turn over at all, they also filled the muffler with corn.
Kids were dinking around one time and took the fuel cap off the tank on a crawler loader sitting in the woods one fall and in about a weeks time squirrels filled the fuel tank full of acorns so when the owner came back to use it and fuel it, the tank was full to the top with acorns and he coulnd't even get the fuel nozzle in the tank, after that ordeal he went to locking the fuel caps with a paddle lock.
Same guy with the combine engine filled with corn called me one day to ask for help getting his tractor going, turns out his grandkids were playing on it weeks before and took the gas cap off the gas tank, shined a light down inside to see what stunk so badly and found several dead mice floating and some decayed at the bottom and the whole tank had to come off to clean it out.
Years back when diesel pickups first came out,[god I feel old] walked into the dealership one day needing parts and all the mechanics were working on a brand new pickup in the shop, turns out someone dumped acid in the guys fuel tank and he had started the pickup up not knowing it and it quit about a mile from his house, he ended up that time putting a new fuel tank on, all new lines and a whole new pump if i recall, and warranty didin't cover it, the truck at the time had less than a 1000 miles on it and the discussion was who and how they were going to tell the owner what happened to his new pickup when he came back to get it after his short trip to the coffee shop while waiting for his truck to get fixed.