Well, I wish a previous tenant on my plot hadn't spread foam backed blue carpet all over the place, so I am with Victoria Park on that one, it had couch grass and horsetail all the way through it. Mainly though I wish they hadn't smashed up an old greenhouse in situ and covered it with soil.
I also wish a former next door neighbour hadn't diverted the boundary fence in order to acquire an apple tree that was actually in our plot, plus a good chunk of plot too - now corrected once he left.