I guess I've been lucky. The only thing I've found in my field besides weeds is holes. Found a spot where (I assume) one of my idiot neighbors buried a truck up to it's frame in the mud a few years prior. After hitting this spot 3 times, & taking out the shear bolt on the mower 3 times,

I filled & graded it. I know this was done by a stuck truck because you could see the tracks in & out on satellite photos!

Found a couple other holes (by "holes" I mean areas about 6'X10'X almost 1' deep) close to the idiot neighbor's place. These didn't take out the shear bolt, but would scare me when I fell in them. My brother was watching from the back deck while I filled those. He was surprised, & shocked, that it took me 3-4 FULL buckets of fill each before he saw any of the fill after I dumped it.
Oh, the only other thing worth mentioning found while mowing the field was just last month when I discovered a large area of raw septic run-off, again from the idiot neighbor's place! Called the county health inspector first thing Monday morning, a couple days later he came out, looked it over, & had a talk with the neighbors. A week later one of the local septic guy's truck was parked there for hours. I guess it'll be another week or so before I get back out there to see if the problem has been addressed.
BTW, the reason I call them "idiot neighbors" is because their land survey shows a PROPOSED 50' right-of-way
NEXT to their property. They assumed this gave them the right to use this 50' piece as their own. They removed our survey stakes, TWICE, then the woman that lives there cursed (like a drunken sailor) the guys working for me when they were setting the survey stakes back in place again in preparation for installing a fence. Police were called, yadda, yadda, yadda, then it took more than a year before the township attorney would say, "That is YOUR property line, you CAN put up your fence there." (see fence in attachment)